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INDEX TO FINANCIAL STATEMENTS

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Filed pursuant to Rule 497
Registration Statement No. 333-155806

PROSPECTUS SUPPLEMENT
(to Prospectus dated June 17, 2010)

2,800,000 Shares

Main Street Capital Corporation

Common Stock

          We are offering for sale 2,800,000 shares of our common stock.

          We are a principal investment firm primarily focused on providing customized debt and equity financing to lower middle-market companies that operate in diverse industries. We seek to fill the current financing gap for lower middle-market businesses, which have more limited access to financing from commercial banks and other traditional sources. Our principal investment objective is to maximize our portfolio's total return by generating current income from our debt investments and capital appreciation from our equity and equity-related investments, including warrants, convertible securities and other rights to acquire equity securities in a portfolio company.

          In addition to our core investment strategy of investing in lower middle market companies, we opportunistically pursue investments in privately placed debt securities. This private placement investment portfolio primarily consists of direct or secondary private placements of interest-bearing securities in companies that are generally larger in size than the lower middle market companies included in our core portfolio.

          We are an internally managed, closed-end, non-diversified management investment company that has elected to be treated as a business development company under the Investment Company Act of 1940.

          Our common stock is listed on the Nasdaq Global Select Market under the symbol "MAIN." On August 16, 2010, the last reported sale price of our common stock on the Nasdaq Global Select Market was $15.62 per share.

          Investing in our common stock involves a high degree of risk, and should be considered highly speculative. See "Risk Factors" beginning on page 13 of the accompanying prospectus to read about factors you should consider, including the risk of leverage, before investing in our common stock.

          This prospectus supplement and the accompanying prospectus contain important information about us that a prospective investor should know before investing in our common stock. Please read this prospectus supplement and the accompanying prospectus before investing and keep them for future reference. We file annual, quarterly and current reports, proxy statements and other information with the Securities and Exchange Commission, or SEC. This information is available free of charge by contacting us at 1300 Post Oak Boulevard, Suite 800, Houston, Texas 77056 or by telephone at (713) 350-6000 or on our website at www.mainstcapital.com. Information contained on our website is not incorporated by reference into this prospectus supplement or the accompanying prospectus, and you should not consider that information to be part of this prospectus supplement or the accompanying prospectus. The SEC also maintains a website at www.sec.gov that contains such information.

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We estimate that we will incur approximately $200,000 in offering expenses in connection with this offering.

          The underwriters have the option to purchase up to an additional 420,000 shares of common stock at the public offering price, less the underwriting discount, within 30 days from the date of this prospectus supplement solely to cover any over-allotments. If the over-allotment option is exercised in full, the total public offering price will be $48,300,000, and the total underwriting discount (4.75%) will be $2,294,250. The proceeds to us would be $46,005,750, before deducting estimated expenses payable by us of $200,000.

          Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of these securities or determined if this prospectus supplement or the accompanying prospectus is truthful or complete. Any representation to the contrary is a criminal offense.

          The underwriters expect to deliver the shares on or about August 19, 2010.

Morgan Keegan & Company, Inc.

The date of this prospectus supplement is August 17, 2010

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ABOUT THE PROSPECTUS

        This document is in two parts. The first part is this prospectus supplement, which describes the terms of this offering of common stock and also adds to and updates information contained in the accompanying prospectus. The second part is the accompanying prospectus, which provides more information about the common stock we may offer from time to time. To the extent the information contained in this prospectus supplement differs from the information contained in the accompanying prospectus, the information in this prospectus supplement shall control.

        You should rely only on the information contained in this prospectus supplement and the accompanying prospectus. Neither we nor the underwriters have authorized any other person to provide you with different information from that contained in this prospectus supplement or the accompanying prospectus. If anyone provides you with different or inconsistent information, you should not rely on it. This prospectus supplement and the accompanying prospectus do not constitute an offer to sell, or a solicitation of an offer to buy, any shares of our common stock by any person in any jurisdiction where it is unlawful for that person to make such an offer or solicitation or to any person in any jurisdiction to whom it is unlawful to make such an offer or solicitation. The information contained in this prospectus supplement and the accompanying prospectus is complete and accurate only as of their respective dates, regardless of the time of their delivery or sale of our common stock. This prospectus supplement supersedes the accompanying prospectus to the extent it contains information different from or additional to the information in that prospectus.

Forward-Looking Statements

        Information contained in this prospectus supplement and the accompanying prospectus may contain forward-looking statements, which can be identified by the use of forward-looking terminology such as "may," "will," "expect," "intend," "anticipate," "estimate," or "continue" or the negative thereof or other variations thereon or comparable terminology. The matters described in the section titled "Risk Factors" in the accompanying prospectus and certain other factors noted throughout this prospectus supplement and the accompanying prospectus constitute cautionary statements identifying important factors with respect to any such forward-looking statements, including certain risks and uncertainties that could cause actual results to differ materially from those in such forward-looking statements. We undertake no obligation to revise or update any forward-looking statements but advise you to consult any additional disclosures that we may make directly to you or through reports that we may file in the future with the SEC, including annual reports on Form 10-K, quarterly reports on Form 10-Q and current reports on Form 8-K. We note that the safe harbor for forward-looking statements provided by the Private Securities Litigation Reform Act of 1995 does not apply to statements made in this prospectus supplement.

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PROSPECTUS SUMMARY

        This summary highlights some of the information in this prospectus supplement and the accompanying prospectus. It is not complete and may not contain all of the information that you may want to consider. To understand the terms of the common stock offered hereby, you should read the entire prospectus supplement and the accompanying prospectus carefully. Together, these documents describe the specific terms of the shares we are offering. You should carefully read the sections titled "Selected Financial Data," "Interim Management's Discussion and Analysis of Financial Condition and Results of Operations" and "Interim Financial Statements" and the documents identified in the section titled "Available Information" in this prospectus supplement, as well as the section titled "Risk Factors" in the accompanying prospectus. Except as otherwise noted, all information in this prospectus supplement and the accompanying prospectus assumes no exercise of the underwriters' over-allotment option.

Organization

        Main Street Capital Corporation ("MSCC") was formed on March 9, 2007 for the purpose of (i) acquiring 100% of the equity interests of Main Street Mezzanine Fund, LP ("MSMF") and its general partner, Main Street Mezzanine Management, LLC ("MSMF GP"), (ii) acquiring 100% of the equity interests of Main Street Capital Partners, LLC (the "Investment Manager"), (iii) raising capital in an initial public offering, which was completed in October 2007 (the "IPO"), and (iv) thereafter operating as an internally managed business development company ("BDC") under the Investment Company Act of 1940, as amended (the "1940 Act"). MSMF is licensed as a Small Business Investment Company ("SBIC") by the United States Small Business Administration ("SBA") and the Investment Manager acts as MSMF's manager and investment adviser. The IPO and related transactions discussed above were consummated in October 2007 and are collectively termed the "Formation Transactions."

        On January 7, 2010, MSCC consummated transactions (the "Exchange Offer") to exchange 1,239,695 shares of its common stock for approximately 88% of the total dollar value of the limited partner interests in Main Street Capital II, LP ("MSC II" and, together with MSMF, the "Funds"). Pursuant to the terms of the Exchange Offer, 100% of the membership interests in the general partner of MSC II, Main Street Capital II GP, LLC ("MSC II GP"), were also transferred to MSCC for no consideration. MSC II commenced operations in January 2006, is an investment fund that operates as an SBIC and is also managed by the Investment Manager. The Exchange Offer and related transactions, including the transfer of the MSC II GP interests, are collectively termed the "Exchange Offer Transactions." As of June 30, 2010, an approximately 12% minority ownership in the total dollar value of the MSC II limited partnership interests remains outstanding, including approximately 5% owned by affiliates of MSCC.

        MSCC has direct or indirect subsidiaries that have elected to be taxable entities (the "Taxable Subsidiaries"). The primary purpose of these entities is to hold certain investments that generate "pass through" income for tax purposes. The Taxable Subsidiaries are each taxed at their normal corporate tax rates based on their taxable income.

        Unless otherwise noted or the context otherwise indicates, the terms "we," "us," "our" and "Main Street" refer to MSCC and its subsidiaries, including MSMF, MSC II, and the Taxable Subsidiaries.

Overview

        We are a principal investment firm primarily focused on providing customized debt and equity financing to lower middle market companies, which we generally define as companies with annual revenues between $10 million and $100 million that operate in diverse industries. We invest primarily in secured debt instruments, equity investments, warrants and other securities of lower middle market companies based in the United States. Our principal investment objective is to maximize our portfolio's total return by generating current income from our debt investments and capital appreciation from our

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equity and equity-related investments, including warrants, convertible securities and other rights to acquire equity securities in a portfolio company. Our core portfolio investments generally range in size from $3 million to $20 million.

        Our investments are generally made through MSCC and the Funds. MSCC and the Funds share the same investment strategies and criteria, although they are subject to different regulatory regimes. An investor's return in MSCC will depend, in part, on the Funds' investment returns as MSMF is a wholly owned subsidiary of MSCC and MSC II is a majority owned subsidiary of MSCC.

        We seek to fill the current financing gap for lower middle market businesses, which, historically, have had more limited access to financing from commercial banks and other traditional sources. Given the current credit environment, we believe the limited access to financing for lower middle market companies is even more pronounced. The underserved nature of the lower middle market creates the opportunity for us to meet the financing needs of lower middle market companies while also negotiating favorable transaction terms and equity participations. Our ability to invest across a company's capital structure, from senior secured loans to equity securities, allows us to offer portfolio companies a comprehensive suite of financing solutions, or "one stop" financing. Providing customized, "one stop" financing solutions has become even more relevant to our portfolio companies in the current credit environment. We generally seek to partner directly with entrepreneurs, management teams and business owners in making our investments. Main Street believes that its core investment strategy has a lower correlation to the broader debt and equity markets.

        As of June 30, 2010, we had debt and equity investments in 41 core portfolio companies with an aggregate fair value of $252.7 million and the weighted average effective yield on our debt investments was approximately 14.7%. Approximately 78% of our total core portfolio investments at cost were in the form of debt investments and 86% of such debt investments at cost were secured by first priority liens on the assets of our portfolio companies as of June 30, 2010. At June 30, 2010, we had equity ownership in approximately 90% of our core portfolio companies and the average fully diluted equity ownership in those portfolio companies was approximately 35%.

        In addition to our core investment strategy of investing in lower middle market companies, we opportunistically pursue investments in privately placed debt securities. This privately placed investment portfolio primarily consists of direct or secondary private placements of interest-bearing securities in companies that are generally larger in size than the lower middle market companies included in our core portfolio. At June 30, 2010, we had privately placed portfolio investments in 10 companies collectively totaling approximately $52.8 million in fair value with a total cost basis of approximately $52.4 million. The median revenues for the 10 privately placed portfolio company investments was approximately $239 million. All of our privately placed portfolio investments at cost were in the form of secured debt investments and 83% of such debt investments at cost were secured by first priority liens on portfolio company assets. The weighted average effective yield on our privately placed portfolio debt investments was approximately 13.5% as of June 30, 2010. Weighted average effective yields are computed using the effective interest rates for all debt investments at June 30, 2010, including amortization of deferred debt origination fees and accretion of original issue discount but excluding any debt investments on non-accrual status and any liquidation fees payable upon repayment.

        Our principal executive offices are located at 1300 Post Oak Boulevard, Suite 800, Houston, Texas 77056, and our telephone number is (713) 350-6000. We maintain a website at http://www.mainstcapital.com. Information contained on our website is not incorporated by reference into this prospectus supplement or the accompanying prospectus, and you should not consider that information to be part of this prospectus supplement or the accompanying prospectus.

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Recent Developments

        On July 1, 2010, our Board of Directors approved the issuance of 149,357 shares of restricted stock to our employees pursuant to the Main Street Capital Corporation 2008 Equity Incentive Plan. These shares will vest over a four-year period from the grant date and will be expensed over a four-year service period starting on the grant date.

        On July 1, 2010, a total of 7,920 shares of restricted stock was issued to our independent directors pursuant to the Main Street Capital Corporation 2008 Non-Employee Director Restricted Stock Plan. These shares will vest on the day immediately preceding the next annual meeting at which our stockholders elect directors, provided that these independent directors have been in continuous service as members of the Board through such date. As a result, these shares will be expensed over a one-year service period starting on the grant date.

        On July 8, 2010, we exited our debt and equity investment in Advantage Millwork Company, Inc., one of the portfolio companies on non-accrual status as of June 30, 2010, at a price equal to such investment's fair value at June 30, 2010. For the third quarter of 2010, we recognized a realized loss of approximately $1.9 million on the exit of this investment.

        On July 30, 2010, we exited our largest private placement debt investment in Apria Healthcare Group, Inc., at a price exceeding the portfolio company's fair value at June 30, 2010. Therefore, we recognized a $0.5 million realized gain in the third quarter of 2010 related to the exit of this investment. Subsequent to exiting this investment, we repaid all debt borrowed under our $30 million investment credit facility, and we currently have no outstanding debt under such facility.

The Offering

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FEES AND EXPENSES

        The following table is intended to assist you in understanding the costs and expenses that an investor in this offering will bear directly or indirectly. We caution you that some of the percentages indicated in the table below are estimates and may vary. Except where the context suggests otherwise, whenever this prospectus supplement contains a reference to fees or expenses paid by "you," "us" or "Main Street," or that "we" will pay fees or expenses, stockholders will indirectly bear such fees or expenses as investors in us.

Example

        The following example demonstrates the projected dollar amount of total cumulative expenses that would be incurred over various periods with respect to a hypothetical investment in our common stock. In calculating the following expense amounts, we have assumed we would have no additional leverage and that our annual operating expenses would remain at the levels set forth in the table above, and that you would pay a sales load of 4.75% (the underwriting discount to be paid by us with respect to common stock sold by us in this offering).

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        The example and the expenses in the table above should not be considered a representation of our future expenses, and actual expenses may be greater or less than those shown. While the example assumes, as required by the SEC, a 5.0% annual return, our performance will vary and may result in a return greater or less than 5.0%. In addition, while the example assumes reinvestment of all dividends at net asset value, participants in our dividend reinvestment plan will receive a number of shares of our common stock, determined by dividing the total dollar amount of the dividend payable to a participant by (i) the market price per share of our common stock at the close of trading on the dividend payment date in the event that we use newly issued shares to satisfy the share requirements of the divided reinvestment plan or (ii) the average purchase price of all shares of common stock purchased by the administrator of the dividend reinvestment plan in the event that shares are purchased in the open market to satisfy the share requirements of the dividend reinvestment plan, which may be at, above or below net asset value. See "Dividend Reinvestment Plan" in the accompanying prospectus for additional information regarding our dividend reinvestment plan.

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USE OF PROCEEDS

        The net proceeds from the sale of the 2,800,000 shares of common stock in this offering are $39,805,000, and $45,805,750 if the underwriter's over-allotment option is exercised in full, after deducting the underwriting discount and estimated offering expenses of approximately $200,000 payable by us.

        We intend to use all of the net proceeds from this offering to make investments in accordance with our investment objective and strategies described in this prospectus supplement and the accompanying prospectus, pay our operating expenses and other cash obligations, and for general corporate purposes. Pending such uses, we currently intend to invest the net proceeds of this offering in marketable securities and idle funds investments, which may include investments in secured intermediate term bank debt, rated debt securities and other income producing investments, consistent with our BDC election and our election to be taxed as a RIC. See "Regulation—Regulation as a Business Development Company—Marketable Securities and Idle Funds Investments" in the accompanying prospectus.

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CAPITALIZATION

        The following table sets forth our capitalization:

•

on an actual basis as of June 30, 2010; and



•

on an as-adjusted basis giving effect to the sale of 2,800,000 shares of our common stock in this offering at the public offering price of $15.00 per share, less estimated underwriting discounts and offering expenses payable by us.

        This table should be read in conjunction with "Interim Management's Discussion and Analysis of Financial Condition and Results of Operations" and "Interim Financial Statements" in this prospectus supplement.

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SELECTED FINANCIAL DATA

        The selected financial data below reflects the combined operations of MSMF and MSMF GP for the years ended December 31, 2005 and 2006 and the consolidated operations of Main Street and its subsidiaries for the years ended December 31, 2007, 2008 and 2009 and the six months ended June 30, 2009 and 2010. The selected financial data at December 31, 2005, 2006, 2007, 2008 and 2009, and for the years ended December 31, 2005, 2006, 2007, 2008 and 2009, have been derived from combined/consolidated financial statements that have been audited by Grant Thornton LLP, an independent registered public accounting firm. The selected financial data for the six months ended June 30, 2009 and 2010, and as of June 30, 2010, have been derived from unaudited financial data but, in the opinion of management, reflects all adjustments (consisting only of normal recurring adjustments) that are necessary to present fairly the results for such interim periods. Interim results as of and for the six months ended June 30, 2010 are not necessarily indicative of the results that may be expected for the year ending December 31, 2010. You should read this selected financial data in conjunction with our "Management's Discussion and Analysis of Financial Condition and Results of Operations," "Senior Securities" and the financial statements and related notes thereto in the accompanying prospectus and "Interim Management's Discussion and Analysis of Financial Condition and Results of Operations" and "Interim Financial Statements" in this prospectus supplement.

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ADDITIONAL MATERIAL U.S. FEDERAL INCOME TAX CONSIDERATIONS

        As described more fully in the accompanying prospectus, we have elected to be taxed as a RIC under Subchapter M of the Code and the applicable Treasury Regulations, which set forth the requirements for qualification as a RIC. The following discussion, which supplements and updates the discussion under the heading "Material U.S. Federal Income Tax Considerations" in the accompanying prospectus, is a summary of certain additional material U.S. federal income tax considerations. You are urged to consult your own tax advisor regarding the specific tax consequences of the purchase, ownership and sale of our common stock.

Sunset of Reduced Tax Rate Provisions

        Several of the tax considerations described under the heading "Material U.S. Federal Income Tax Considerations" in the accompanying prospectus are subject to sunset provisions. These sunset provisions generally provide that for taxable years beginning after December 31, 2010, certain provisions in the Code that are currently applicable will revert back to earlier versions of such provisions. As a result, the federal income tax rates applicable to ordinary income, long-term capital gain and qualified dividend income for taxpayers taxed at individual rates will increase beginning January 1, 2011, absent congressional action. Consequently, prospective investors should consult their own tax advisors regarding the effect of the sunset provisions on an investment in our common stock.

Recent Tax Legislation

        On March 18, 2010, the President signed into law the Hiring Incentives to Restore Employment Act of 2010, or the HIRE Act. The HIRE Act will require, after December 31, 2012, withholding at a rate of 30% on dividends in respect of, and gross proceeds from the sale or other disposition of, shares of our common stock held by "foreign financial institutions" (including foreign investment funds), unless such institution enters into an agreement with the Secretary of the Treasury to report, on an annual basis, information about equity and debt interests in, and accounts maintained by, the institution to the extent such interests or accounts are held by certain U.S. persons or by certain non-U.S. entities that are wholly or partially owned by U.S. persons. Similarly, after December 31, 2012, dividends in respect of, and gross proceeds from the sale or other disposition of, shares of our common stock held by an investor that is a non-financial foreign entity will be subject to withholding at a rate of 30%, unless such entity either (i) certifies to us that such entity does not have any "substantial United States owners" or (ii) provides certain information regarding the entity's "substantial United States owners," which we will in turn provide to the Secretary of the Treasury.

        On March 30, 2010, the President signed into law the Health Care and Education Reconciliation Act of 2010, or the Reconciliation Act. The Reconciliation Act will require that certain U.S. stockholders who are individuals, estates or trusts to pay a 3.8% Medicare tax on, among other things, dividends on and capital gains from the sale or other disposition of stock, subject to certain exceptions. This tax will apply for taxable years beginning after December 31, 2012.

        Prospective investors are encouraged to consult with their tax advisors regarding the possible implications of the recent legislation described herein on an investment in our common stock.

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UNDERWRITING

        Under the terms and subject to the conditions contained in an underwriting agreement dated August 17, 2010, the underwriters named below, for whom Morgan Keegan & Company, Inc. is acting as representative, have severally agreed to purchase, and we have agreed to sell to them, the number of shares of common stock indicated below:

        The underwriting agreement provides that the obligations of the underwriters to pay for and accept delivery of the shares of common stock offered hereby are subject to the approval of certain legal matters by their counsel and to certain other conditions. The underwriters are severally obligated to take and pay for all shares of common stock offered hereby (other than those covered by the underwriters' over-allotment option described below) if any such shares are taken. We have agreed to indemnify the underwriters against certain liabilities, including liabilities under the Securities Act.

        Our common stock is listed on the Nasdaq Global Select Market under the symbol "MAIN."

Over-Allotment Option

        We have granted to the underwriters an option, exercisable for 30 days from the date of this prospectus supplement, to purchase up to an aggregate of 420,000 additional shares of common stock at the public offering price set forth on the cover page hereof, less the underwriting discount. The underwriters may exercise this option solely for the purpose of covering over-allotments, if any, made in connection with the offering of the shares of common stock offered hereby. To the extent such option is exercised, each underwriter will become obligated, subject to certain conditions, to purchase approximately the same percentage of such additional shares of common stock as the number set forth next to such underwriter's name in the preceding table bears to the total number of shares set forth next to the names of all underwriters in the preceding table.

Lock-Up Agreements

        We, and certain of our executive officers and directors, have agreed, subject to certain exceptions, not to issue, sell, offer to sell, contract or agree to sell, hypothecate, pledge, transfer, grant any option to purchase, establish an open put equivalent position or otherwise dispose of or agree to dispose of directly or indirectly, any shares of our common stock, or any securities convertible into or exercisable or exchangeable for any shares of our common stock or any right to acquire shares of our common stock, for 60 days from the date of this prospectus supplement, subject to extension upon material announcements or earnings releases. The representative, at any time and without notice, may release all or any portion of the common stock subject to the foregoing lock-up agreements.

Underwriting Discounts

        The underwriters initially propose to offer the shares directly to the public at the public offering price set forth on the cover page of this prospectus supplement and to certain dealers at a price that represents a concession not in excess of $0.43 per share below the public offering price. After the initial public offering of the shares, the offering price and other selling terms may be changed by the underwriters.

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        The following table provides information regarding the per share and total underwriting discount that we are to pay to the underwriters. These amounts are shown assuming both no exercise and full exercise of the underwriters' option to purchase up to 420,000 additional shares from us.

        We will pay all expenses incident to the offering and sale of shares of our common stock by us in this offering. We estimate that the total expenses of the offering, excluding the underwriting discount will be approximately $200,000.

        A prospectus supplement in electronic format may be made available on the web sites maintained by one or more of the underwriters, or selling group members, if any, participating in this offering. The representative may agree to allocate a number of shares to underwriters and selling group members for the sale to their online brokerage account holders. Internet distributions will be allocated by the underwriters and selling group members that will make Internet distributions on the same basis as other allocations. The representative may agree to allocate a number of shares to underwriters for sale to their online brokerage account holders.

Price Stabilization, Short Positions and Penalty Bids

        In connection with this offering, the underwriters may purchase and sell shares of our common stock in the open market. These transactions may include over-allotment, syndicate covering transactions and stabilizing transactions. An over-allotment involves syndicate sales of shares in excess of the number of shares to be purchased by the underwriters in the offering, which creates a syndicate short position. Syndicate covering transactions involve purchases of shares in the open market after the distribution has been completed in order to cover syndicate short positions.

        Stabilizing transactions consist of some bids or purchases of shares of our common stock made for the purpose of preventing or slowing a decline in the market price of the shares while the offering is in progress.

        In addition, the underwriters may impose penalty bids, under which they may reclaim the selling concession from a syndicate member when the shares of our common stock originally sold by that syndicate member are purchased in a stabilizing transaction or syndicate covering transaction to cover syndicate short positions.

        Similar to other purchase transactions, these activities may have the effect of raising or maintaining the market price of the common stock or preventing or slowing a decline in the market price of the common stock. As a result, the price of the common stock may be higher than the price that might otherwise exist in the open market. Except for the sale of shares of our common stock in this offering, the underwriters may carry out these transactions on the Nasdaq Global Select Market, in the over-the-counter market or otherwise.

        Neither the underwriters nor we make any representation or prediction as to the direction or magnitude of any effect that the transactions described above may have on the price of the shares. In addition, neither the underwriters nor we make any representation that the underwriters will engage in these transactions or that these transactions, once commenced, will not be discontinued without notice.

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Passive Market Making Pursuant to Regulation M

        In connection with this transaction, the underwriters may engage in passive market making transactions in our common stock on the Nasdaq Global Select Market, prior to the pricing and completion of this offering. Passive market making is permitted by SEC Regulation M and consists of displaying bids on the Nasdaq Global Select Market no higher than the bid prices of independent market makers and making purchases at prices no higher than these independent bids and effected in response to order flow. Net purchases by a passive market maker on each day are limited to a specified percentage of the passive market maker's average daily trading volume in our common stock during a specified period and must be discontinued when such limit is reached. Passive market making may cause the price of our common stock to be higher than the price that otherwise would exist in the open market in the absence of such transactions.

Conflicts of Interest

        Affiliates of BB&T Capital Markets, an underwriter in this offering, act as lenders and/or agents under our $30 million investment credit facility.

        The underwriters and/or their affiliates from time to time provide and may in the future provide investment banking, commercial banking and financial advisory services to us, for which they have received and may receive customary compensation.

        In addition, the underwriters and/or their affiliates may from time to time refer investment banking clients to us as potential portfolio investments. If we invest in those clients, we may utilize net proceeds from this offering to fund such investments, and the referring underwriter or its affiliate may receive placement fees from its client in connection with such financing, which placement fees may be paid out of the amount funded by us.

        The addresses of the underwriters are: Morgan Keegan & Company, Inc., 50 N. Front St., 19th Floor, Memphis, Tennessee 38103; BB&T Capital Markets, 901 East Byrd Street, Suite 410, Richmond, Virginia 23219; Janney Montgomery Scott LLC, 1801 Market Street, Philadelphia, Pennsylvania 19103; and Madison Williams and Company LLC, 527 Madison Ave, New York, New York 10022.

LEGAL MATTERS

        Certain legal matters regarding the shares of common stock offered hereby will be passed upon for us by Sutherland Asbill & Brennan LLP, Washington D.C., and certain legal matters in connection with this offering will be passed upon for the underwriters by Bass, Berry & Sims PLC, Memphis, Tennessee.

INDEPENDENT REGISTERED PUBLIC ACCOUNTING FIRM

        The consolidated financial statements, Schedule 12-14 and the schedule of Senior Securities of Main Street Capital Corporation, included in this prospectus supplement and the accompanying prospectus have been so included in reliance upon the reports of Grant Thornton LLP, independent registered public accountants, upon the authority of said firm as experts in giving said reports. Grant Thornton LLP's principal business address is 333 Clay Street, 2700 Three Allen Center, Houston, Texas 77002.

AVAILABLE INFORMATION

        We have filed with the SEC a registration statement on Form N-2, together with all amendments and related exhibits, under the Securities Act, with respect to our shares of common stock offered by this prospectus supplement. The registration statement contains additional information about us and our shares of common stock being offered by this prospectus supplement.

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        We file with or submit to the SEC annual, quarterly and current reports, proxy statements and other information meeting the informational requirements of the Securities Exchange Act of 1934. You may inspect and copy these reports, proxy statements and other information, as well as the registration statement and related exhibits and schedules, at the Public Reference Room of the SEC at 100 F Street, N.E., Washington, D.C. 20549. You may obtain information on the operation of the Public Reference Room by calling the SEC at 1-800-SEC-0330. The SEC maintains an Internet site that contains reports, proxy and information statements and other information filed electronically by us with the SEC, which are available on the SEC's website at www.sec.gov. Copies of these reports, proxy and information statements and other information may be obtained, after paying a duplicating fee, by electronic request at the following e-mail address: publicinfo@sec.gov, or by writing the SEC's Public Reference Section, 100 F Street, N.E., Washington, D.C. 20549.

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INTERIM MANAGEMENT'S DISCUSSION AND ANALYSIS OF
FINANCIAL CONDITION AND RESULTS OF OPERATIONS

        The following discussion should be read in conjunction with "Interim Financial Statements" in this prospectus supplement.

        Statements we make in the following discussion which express a belief, expectation or intention, as well as those that are not historical fact, are forward-looking statements that are subject to risks, uncertainties and assumptions. Our actual results, performance or achievements, or industry results, could differ materially from those we express in the following discussion as a result of a variety of factors, including the risks and uncertainties we have referred to under the headings "Cautionary Statement Concerning Forward-Looking Statements" and "Risk Factors" in the accompanying prospectus.

ORGANIZATION

        Main Street Capital Corporation ("MSCC") was formed on March 9, 2007 for the purpose of (i) acquiring 100% of the equity interests of Main Street Mezzanine Fund, LP ("MSMF") and its general partner, Main Street Mezzanine Management, LLC ("MSMF GP"), (ii) acquiring 100% of the equity interests of Main Street Capital Partners, LLC (the "Investment Manager"), (iii) raising capital in an initial public offering, which was completed in October 2007 (the "IPO"), and (iv) thereafter operating as an internally managed business development company ("BDC") under the Investment Company Act of 1940, as amended (the "1940 Act"). MSMF is licensed as a Small Business Investment Company ("SBIC") by the United States Small Business Administration ("SBA") and the Investment Manager acts as MSMF's manager and investment adviser. Because the Investment Manager, which employs all of the executive officers and other employees of MSCC, is wholly owned by us, we do not pay any external investment advisory fees, but instead we incur the net operating costs associated with employing investment and portfolio management professionals through the Investment Manager. The IPO and related transactions discussed above were consummated in October 2007 and are collectively termed the "Formation Transactions."

        On January 7, 2010, MSCC consummated transactions (the "Exchange Offer") to exchange 1,239,695 shares of its common stock for approximately 88% of the total dollar value of the limited partner interests in Main Street Capital II, LP ("MSC II" and, together with MSMF, the "Funds"). Pursuant to the terms of the Exchange Offer, 100% of the membership interests in the general partner of MSC II, Main Street Capital II GP, LLC ("MSC II GP"), were also transferred to MSCC for no consideration. MSC II commenced operations in January 2006, is an investment fund that operates as an SBIC and is also managed by the Investment Manager. The Exchange Offer and related transactions, including the transfer of the MSC II GP interests, are collectively termed the "Exchange Offer Transactions" (see Note I to the consolidated financial statements). As of June 30, 2010, an approximately 12% minority ownership in the total dollar value of the MSC II limited partnership interests remains outstanding, including approximately 5% owned by affiliates of MSCC.

        MSCC has direct or indirect subsidiaries that have elected to be taxable entities (the "Taxable Subsidiaries"). The primary purpose of these entities is to hold certain investments that generate "pass through" income for tax purposes. The Taxable Subsidiaries are each taxed at their normal corporate tax rates based on their taxable income.

        Unless otherwise noted or the context otherwise indicates, the terms "we," "us," "our" and "Main Street" refer to MSCC and its subsidiaries, including MSMF, MSC II, and the Taxable Subsidiaries.

OVERVIEW

        We are a principal investment firm primarily focused on providing customized debt and equity financing to lower middle market companies, which we generally define as companies with annual

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revenues between $10 million and $100 million that operate in diverse industries. We invest primarily in secured debt instruments, equity investments, warrants and other securities of lower middle market companies based in the United States. Our principal investment objective is to maximize our portfolio's total return by generating current income from our debt investments and capital appreciation from our equity and equity-related investments, including warrants, convertible securities and other rights to acquire equity securities in a portfolio company. Our core portfolio investments generally range in size from $3 million to $20 million.

        Our investments are generally made through MSCC and the Funds. MSCC and the Funds share the same investment strategies and criteria, although they are subject to different regulatory regimes. An investor's return in MSCC will depend, in part, on the Funds' investment returns as MSMF is a wholly owned subsidiary of MSCC and MSC II is a majority owned subsidiary of MSCC.

        We seek to fill the current financing gap for lower middle market businesses, which, historically, have had more limited access to financing from commercial banks and other traditional sources. Given the current credit environment, we believe the limited access to financing for lower middle market companies is even more pronounced. The underserved nature of the lower middle market creates the opportunity for us to meet the financing needs of lower middle market companies while also negotiating favorable transaction terms and equity participations. Our ability to invest across a company's capital structure, from senior secured loans to equity securities, allows us to offer portfolio companies a comprehensive suite of financing solutions, or "one stop" financing. Providing customized, "one stop" financing solutions has become even more relevant to our portfolio companies in the current credit environment. We generally seek to partner directly with entrepreneurs, management teams and business owners in making our investments. Main Street believes that its core investment strategy has a lower correlation to the broader debt and equity markets.

        In addition to our core investment strategy of investing in lower middle market companies, we opportunistically pursue investments in privately placed debt securities. This private placement investment portfolio primarily consists of direct or secondary private placements of interest-bearing securities in companies that are generally larger in size than the lower middle market companies included in our core portfolio. At June 30, 2010, we had privately placed portfolio investments in 10 companies collectively totaling approximately $52.8 million in fair value with a total cost basis of approximately $52.4 million. The median revenues for the 10 privately placed portfolio company investments was approximately $239 million. All of our total privately placed portfolio investments at cost were in the form of secured debt investments and 83% of such debt investments at cost were secured by first priority liens on portfolio company assets. The weighted average effective yield on our privately placed portfolio debt investments was approximately 13.5% as of June 30, 2010.

        The level of new portfolio investment activity will fluctuate from period to period based upon our view of the current economic fundamentals, our ability to identify new investment opportunities that meet our investment criteria, and our ability to consummate identified opportunities. The level of new investment activity, and associated interest and fee income, will directly impact future investment income. In addition, the level of dividends paid by portfolio companies and the portion of our portfolio debt investments on non-accrual status will directly impact future investment income. While we intend to grow our portfolio and our investment income over the long-term, our growth and our operating results may be more limited during depressed economic periods. However, we intend to appropriately manage our cost structure and liquidity position based on applicable economic conditions and our investment outlook. The level of realized gains or losses and unrealized appreciation or depreciation will also fluctuate depending upon portfolio activity and the performance of our individual portfolio companies. The changes in realized gains and losses and unrealized appreciation or depreciation could have a material impact on our operating results.

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        For the six months ended June 30, 2010, we paid dividends on a monthly basis totaling $0.75 per share, or $10.8 million. In June 2010, we declared monthly dividends for the third quarter of 2010 totaling $0.375 per share. During 2009, we paid monthly dividends of $0.125 per share, or $1.50 per share for the entire year. We generated undistributed taxable income (or "spillover income") of approximately $0.8 million, or $0.08 per share, during 2009 that was carried forward toward distributions paid in 2010. Including the dividends declared for the third quarter of 2010, we will have paid approximately $4.38 per share in cumulative dividends since our October 2007 initial public offering.

        At June 30, 2010, we had $19.6 million in cash and cash equivalents, marketable securities, and idle funds investments. In January 2010, we completed a follow-on public stock offering in which we sold 2,875,000 shares of common stock, including the underwriters' exercise of the over-allotment option, at a price to the public of $14.75 per share (or approximately 121% of the latest reported Net Asset Value per share), resulting in total net proceeds of approximately $40.1 million, after deducting underwriters' commissions and offering costs. Due to our existing cash, cash equivalents, marketable securities and idle funds investments, and available leverage, we expect to have sufficient cash resources to support our investment and operational activities through the remainder of calendar year 2010. However, this projection will be impacted by, among other things, the pace of new and follow-on investments, debt repayments and investment redemptions, the level of cash flow from operations and cash flow from realized gains, proceeds from future equity offerings, and the level of dividends we pay in cash.

        The American Recovery and Reinvestment Act of 2009 enacted in February 2009 (the "Stimulus Bill") contains several provisions applicable to SBIC funds, including the Funds. One of the key SBIC-related provisions included in the Stimulus Bill increased the maximum amount of combined SBIC leverage (or SBIC leverage cap) to $225 million for affiliated SBIC funds. The prior maximum amount of SBIC leverage available to affiliated SBIC funds was approximately $137 million. Since the increase in the SBIC leverage cap applies to affiliated SBIC funds, Main Street is required to allocate such increased borrowing capacity between the Funds. Main Street has access to an incremental $80 million in SBIC leverage capacity, subject to the required capitalization of each of the Funds, in addition to the $145 million of existing SBIC leverage at the Funds.

        A recently proposed bill, the Small Business Financing and Investment Act of 2009, or HR 3854, would increase the total SBIC leverage capacity for affiliated SBIC funds from $225 million to $350 million. If enacted, this bill would increase Main Street's SBIC leverage capacity through the Funds by an additional $125 million. While Main Street is positioned to benefit from the full congressional passage of HR 3854, the ultimate form and likely outcome of such legislation or any similar legislation cannot be predicted.

        In our view, the SBIC leverage, including the increased capacity, remains a strategic advantage due to its long-term, flexible structure and its low fixed cost. The SBIC leverage also provides proper matching of duration and cost compared with our core portfolio investments. As of June 30, 2010, the weighted average duration of our core portfolio debt investments was approximately 3.3 years compared to a weighted average duration of 6.9 years for our SBIC leverage. As of June 30, 2010, approximately 94% of core portfolio debt investments bear interest at fixed rates which is also appropriately matched by the long-term, low cost fixed rates available through our SBIC leverage.

CRITICAL ACCOUNTING POLICIES

Basis of Presentation

        Our financial statements are prepared in accordance with U.S. Generally Accepted Accounting Principles ("U.S. GAAP"). For the three and six months ended June 30, 2010, our consolidated financial statements include the accounts of MSCC and its consolidated subsidiaries, including MSMF

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and MSC II. For the three and six months ended June 30, 2009, our consolidated financial statements include the accounts of MSCC and its consolidated subsidiaries, including MSMF. To allow for more relevant disclosure of our "core" investment portfolio, "core" portfolio investments, as used herein, refers to all of our portfolio investments in lower middle market companies, excluding the Investment Manager. Private placement portfolio investments include investments made through direct or secondary private placements of interest-bearing securities in companies that are generally larger in size than the lower middle market companies included in our core portfolio. "Marketable securities and idle funds investments" are classified as financial instruments and are reported separately on our Consolidated Balance Sheets and Consolidated Schedule of Investments due to the nature of such investments. Our results of operations and cash flows for the three and six months ended June 30, 2010 and 2009, and financial position as of June 30, 2010 and December 31, 2009, are presented on a consolidated basis. The effects of all intercompany transactions between Main Street and its consolidated subsidiaries have been eliminated in consolidation. Certain reclassifications have been made to prior period balances to conform with the current financial statement presentation, including the reclassification of private placement portfolio investments which were formerly classified as "Marketable securities and idle funds investments" and are now classified as portfolio investments in the "Non-Control/Non-Affiliate investments" category due to our current intent to hold such investments until their maturity and the fact that their terms adhere more to our portfolio investment strategy.

        The accompanying unaudited consolidated financial statements of Main Street are presented in conformity with U.S. GAAP for interim financial information and pursuant to the requirements of Article 10 of Regulation S-X. Accordingly, certain disclosures accompanying annual financial statements prepared in accordance with U.S. GAAP are omitted. In the opinion of management, the unaudited consolidated financial results included herein contain all adjustments, consisting solely of normal recurring accruals, considered necessary for the fair presentation of financial statements for the interim periods included herein. The results of operations for the three and six months ended June 30, 2010 are not necessarily indicative of the operating results to be expected for the full year. Also, the unaudited financial statements and notes should be read in conjunction with the audited financial statements and notes thereto for the year ended December 31, 2009. Financial statements prepared on a U.S. GAAP basis require management to make estimates and assumptions that affect the amounts and disclosures reported in the financial statements and accompanying notes. Such estimates and assumptions could change in the future as more information becomes known, which could impact the amounts reported and disclosed herein.

        Under the investment company rules and regulations pursuant to Article 6 of Regulation S-X and the Audit and Accounting Guide for Investment Companies issued by the American Institute of Certified Public Accountants (the "AICPA Guide"), we are precluded from consolidating portfolio company investments, including those in which we have a controlling interest, unless the portfolio company is another investment company. An exception to this general principle in the AICPA Guide occurs if we own a controlled operating company that provides all or substantially all of its services directly to us, or to an investment company of ours. None of the investments made by us qualify for this exception. Therefore, our portfolio investments are carried on the balance sheet at fair value, as discussed further in Note B to our consolidated financial statements, with any adjustments to fair value recognized as "Net Change in Unrealized Appreciation (Depreciation) from Investments" on our Statement of Operations until the investment is exited, resulting in any gain or loss on exit being recognized as a "Net Realized Gain (Loss) from Investments."

Portfolio Investment Valuation

        The most significant estimate inherent in the preparation of our consolidated financial statements is the valuation of our portfolio investments and the related amounts of unrealized appreciation and

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depreciation. As of June 30, 2010 and December 31, 2009, approximately 92% and 80%, respectively, of our total assets represented investments in portfolio companies valued at fair value (including the investment in the Investment Manager). We are required to report our investments at fair value. We adopted the provisions of Accounting Standards Codification ("Codification" or "ASC") 820, Fair Value Measurements and Disclosures in the first quarter of 2008. ASC 820 defines fair value, establishes a framework for measuring fair value, establishes a fair value hierarchy based on the quality of inputs used to measure fair value, and enhances disclosure requirements for fair value measurements.

        Our core portfolio strategy calls for us to invest primarily in illiquid securities issued by private companies. These core portfolio investments may be subject to restrictions on resale and will generally have no established trading market. As a result, we determine in good faith the fair value of our portfolio investments pursuant to a valuation policy in accordance with ASC 820 and a valuation process approved by our Board of Directors and in accordance with the 1940 Act. We review external events, including private mergers, sales and acquisitions involving comparable companies, and include these events in the valuation process. Our valuation policy and process are intended to provide a consistent basis for determining the fair value of the portfolio.

        For valuation purposes, control investments are composed of equity and debt securities for which we have a controlling interest in the portfolio company or have the ability to nominate a majority of the portfolio company's board of directors. Market quotations are generally not readily available for our control investments. As a result, we determine the fair value of control investments using a combination of market and income approaches. Under the market approach, we will typically use the enterprise value methodology to determine the fair value of these investments. The enterprise value is the fair value at which an enterprise could be sold in a transaction between two willing parties, other than through a forced or liquidation sale. Typically, private companies are bought and sold based on multiples of earnings before interest, taxes, depreciation and amortization, or EBITDA, cash flows, net income, revenues, or in limited cases, book value. There is no single methodology for estimating enterprise value. For any one portfolio company, enterprise value is generally described as a range of values from which a single estimate of enterprise value is derived. In estimating the enterprise value of a portfolio company, we analyze various factors, including the portfolio company's historical and projected financial results. We allocate the enterprise value to investments in order of the legal priority of the investments. We will also use the income approach to determine the fair value of these securities, based on projections of the discounted future free cash flows that the portfolio company or the debt security will likely generate. The valuation approaches for our control investments estimate the value of the investment if we were to sell, or exit, the investment, assuming the highest and best use of the investment by market participants. In addition, these valuation approaches consider the value associated with our ability to control the capital structure of the portfolio company, as well as the timing of a potential exit.

        For valuation purposes, non-control core portfolio investments are composed of debt and equity securities for which we do not have a controlling interest in the portfolio company, or the ability to nominate a majority of the portfolio company's board of directors. Market quotations for non-control investments are generally not readily available. For our non-control core portfolio investments, we use a combination of the market and income approaches to value our equity investments and the income approach to value our debt instruments. For non-control debt investments, we determine the fair value primarily using a yield approach that analyzes the discounted cash flows of interest and principal for the debt security, as set forth in the associated loan agreements, as well as the financial position and credit risk of each of these portfolio investments. Our estimate of the expected repayment date of a debt security is generally the legal maturity date of the instrument, as we generally intend to hold our loans to maturity. The yield analysis considers changes in leverage levels, credit quality, portfolio company performance and other factors. We will use the value determined by the yield analysis as the fair value for that security; however, because of our general intent to hold our loans to maturity, the

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fair value will not exceed the face amount of the debt security. A change in the assumptions that we use to estimate the fair value of our debt securities using the yield analysis could have a material impact on the determination of fair value. If there is deterioration in credit quality or a debt security is in workout status, we may consider other factors in determining the fair value of a debt security, including the value attributable to the debt security from the enterprise value of the portfolio company or the proceeds that would be received in a liquidation analysis.

        Our private placement portfolio investments primarily consist of direct or secondary purchases of interest-bearing securities in companies that are generally larger in size than the lower middle market companies included in our core portfolio. For valuation purposes, all of our private placement portfolio investments are non-control investments and are composed of debt securities for which we do not have a controlling interest in the portfolio company, or the ability to nominate a majority of the portfolio company's board of directors. We primarily use observable inputs to determine the fair value of these investments through obtaining third party quotes or independent pricing.

        Due to the inherent uncertainty in the valuation process, our estimate of fair value may differ materially from the values that would have been used had a ready market for the securities existed. In addition, changes in the market environment, portfolio company performance and other events that may occur over the lives of the investments may cause the gains or losses ultimately realized on these investments to be materially different than the valuations currently assigned. We estimate the fair value of each individual investment and record changes in fair value as unrealized appreciation or depreciation.

Revenue Recognition

Interest and Dividend Income

        We record interest and dividend income on the accrual basis to the extent amounts are expected to be collected. Dividend income is recorded as dividends are declared or at the point an obligation exists for the portfolio company to make a distribution. In accordance with our valuation policy, we evaluate accrued interest and dividend income periodically for collectability. When a loan or debt security becomes 90 days or more past due, and if we otherwise do not expect the debtor to be able to service all of its debt or other obligations, we will generally place the loan or debt security on non-accrual status and cease recognizing interest income on that loan or debt security until the borrower has demonstrated the ability and intent to pay contractual amounts due. If a loan or debt security's status significantly improves regarding ability to service the debt or other obligations, or if a loan or debt security is fully impaired, sold or written off, we will remove it from non-accrual status.

Fee Income

        We may periodically provide services, including structuring and advisory services, to our portfolio companies. For services that are separately identifiable and evidence exists to substantiate fair value, income is recognized as earned, which is generally when the investment or other applicable transaction closes. Fees received in connection with debt financing transactions for services that do not meet these criteria are treated as debt origination fees and are accreted into interest income over the life of the financing.

Payment-in-Kind ("PIK") Interest

        While not significant to its total portfolio, we hold debt and preferred equity instruments in our investment portfolio that contain payment-in-kind ("PIK") interest and cumulative dividend provisions. The PIK interest, computed at the contractual rate specified in each debt agreement, is added to the principal balance of the debt and is recorded as interest income. Thus, the actual collection of this interest may be deferred until the time of debt principal repayment. Cumulative dividends are recorded

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as dividend income, and any dividends in arrears are added to the balance of the preferred equity investment. The actual collection of dividends in arrears may be deferred until such time as the preferred equity is redeemed. To maintain regulated investment company ("RIC") tax treatment (as discussed below), these non-cash sources of income will need to be paid out to stockholders in the form of distributions, even though we may not have collected the PIK interest and cumulative dividends in cash. We will stop accruing PIK interest and cumulative dividends and will write off any accrued and uncollected interest and dividends in arrears when it is determined that such PIK interest and dividends in arrears are no longer collectible.

Share-Based Compensation

        We account for our share-based compensation plans using the fair value method, as prescribed by ASC 718, Compensation—Stock Compensation. Accordingly, for restricted stock awards, we measured the grant date fair value based upon the market price of our common stock on the date of the grant and will amortize this fair value to share-based compensation expense over the requisite service period or vesting term.

Income Taxes

        MSCC has elected and intends to qualify for the tax treatment applicable to a RIC under Subchapter M of the Internal Revenue Code of 1986, as amended (the "Code"), and, among other things, intends to make the required distributions to its stockholders as specified therein. In order to qualify as a RIC, MSCC is required to timely distribute to its stockholders at least 90% of investment company taxable income, as defined by the Code, each year. Depending on the level of taxable income earned in a tax year, MSCC may choose to carry forward taxable income in excess of current year distributions into the next tax year and pay a 4% excise tax on such income. Any such carryover taxable income must be distributed through a dividend declared prior to filing the final tax return related to the year which generated such taxable income.

        The Taxable Subsidiaries hold certain core portfolio investments of Main Street. The Taxable Subsidiaries are consolidated for U.S. GAAP reporting purposes, and the core portfolio investments held by them are included in Main Street's consolidated financial statements. The Taxable Subsidiaries permit Main Street to hold equity investments in portfolio companies which are "pass through" entities for tax purposes in order to comply with the "source income" requirements contained in the RIC tax provisions. The Taxable Subsidiaries are not consolidated with Main Street for income tax purposes and may generate income tax expense as a result of their ownership of certain core portfolio investments. This income tax expense, or benefit, is reflected in Main Street's Consolidated Statement of Operations.

        The Taxable Subsidiaries use the liability method in accounting for income taxes. Deferred tax assets and liabilities are recorded for temporary differences between the tax basis of assets and liabilities and their reported amounts in the financial statements, using statutory tax rates in effect for the year in which the temporary differences are expected to reverse. A valuation allowance is provided against deferred tax assets when it is more likely than not that some portion or all of the deferred tax asset will not be realized.

        Taxable income generally differs from net income for financial reporting purposes due to temporary and permanent differences in the recognition of income and expenses. Taxable income generally excludes net unrealized appreciation or depreciation, as investment gains or losses are not included in taxable income until they are realized.

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CORE PORTFOLIO COMPOSITION

        Core portfolio investments principally consist of secured debt, equity warrants and direct equity investments in privately held companies. The core debt investments are secured by either a first or second lien on the assets of the portfolio company, generally bear interest at fixed rates, and generally mature between five and seven years from the original investment. In most core portfolio companies, we also receive nominally priced equity warrants and/or make direct equity investments, usually in connection with a debt investment.

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        The Investment Manager is a wholly owned subsidiary of MSCC. However, the Investment Manager is accounted for as a portfolio investment of Main Street, since it conducts a significant portion of its investment management activities outside of MSCC and its subsidiaries. To allow for more relevant disclosure of our core investment portfolio, our investment in the Investment Manager has been excluded from the tables and amounts set forth below.

        Summaries of the composition of our core investment portfolio at cost and fair value as a percentage of total core portfolio investments are shown in the following table:

        The following table shows the core portfolio composition by geographic region of the United States at cost and fair value as a percentage of total core portfolio investments. The geographic composition is determined by the location of the corporate headquarters of the portfolio company:

        Main Street's core portfolio investments are generally in lower middle market companies conducting business in a variety of industries. Set forth below are tables showing the composition of

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Main Street's core portfolio by industry at cost and fair value as of June 30, 2010 and December 31, 2009:

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        Our core portfolio investments carry a number of risks including, but not limited to: (1) investing primarily in lower middle market companies which may have a limited operating history and financial resources; (2) holding investments that are not publicly traded and which may be subject to legal and other restrictions on resale; and (3) other risks common to investing in below investment grade debt and equity investments in private, lower middle market companies.

PORTFOLIO ASSET QUALITY

        We utilize an internally developed investment rating system to rate the performance of each portfolio company. Investment Rating 1 represents a portfolio company that is performing in a manner which significantly exceeds expectations and projections. Investment Rating 2 represents a portfolio company that, in general, is performing above expectations. Investment Rating 3 represents a portfolio company that is generally performing in accordance with expectations. Investment Rating 4 represents a portfolio company that is underperforming expectations. Investments with such a rating require increased monitoring and scrutiny by us. Investment Rating 5 represents a portfolio company that is significantly underperforming. Investments with such a rating require heightened levels of monitoring and scrutiny by us and involve the recognition of significant unrealized depreciation on such investment. All new portfolio investments receive an initial 3 rating.

        The following table shows the distribution of our portfolio investments (excluding the investment in the Investment Manager) on our 1 to 5 investment rating scale at fair value as of June 30, 2010 and December 31, 2009:

        Based upon our investment rating system, the weighted average rating of our portfolio as of June 30, 2010 and December 31, 2009 was approximately 2.4. As of June 30, 2010, we had three investments on non-accrual status, which comprised approximately 3.7% of the investment portfolio at fair value. On July 8, 2010, we exited our debt and equity investment in Advantage Millwork Company, Inc., one of the portfolio companies on non-accrual status as of June 30, 2010, at a price equal to the portfolio company's fair value at June 30, 2010. At December 31, 2009, we had three investments on non-accrual status, which comprised approximately 1.1% of the investment portfolio at fair value.

        The broader fundamentals of the United States economy remain mixed, and unemployment remains elevated. In the event that the United States economy contracts, it is likely that the financial results of small- to mid-sized companies, like those in which we invest, could experience deterioration or limited growth from current levels, which could ultimately lead to difficulty in meeting their debt service requirements and an increase in defaults. In addition, the end markets for certain of our portfolio companies' products and services have experienced negative economic trends over the last few years. Consequently, we can provide no assurance that the performance of certain portfolio companies will not be negatively impacted by these economic or other conditions, which could also have a negative impact on our future results.

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DISCUSSION AND ANALYSIS OF RESULTS OF OPERATIONS

Comparison of the three months ended June 30, 2010 and June 30, 2009