QuickLinks -- Click here to rapidly navigate through this document


Exhibit 32.1

Certification of Chief Executive Officer
Pursuant to
Section 906 of the Sarbanes-Oxley Act of 2002 (18 U.S.C. 1350)

        In connection with the Quarterly Report of Main Street Capital Corporation (the "Registrant") on Form 10-Q for the quarter ended September 30, 2015 (the "Report"), as filed with the Securities and Exchange Commission on the date hereof, I, Vincent D. Foster, the Chairman and Chief Executive Officer of the Registrant, hereby certify, pursuant to 18 U.S.C. §1350, as adopted pursuant to §906 of the Sarbanes-Oxley Act of 2002, that:

  /s/ VINCENT D. FOSTER

  Name:   Vincent D. Foster

  Date:   November 6, 2015



QuickLinks

Certification of Chief Executive Officer Pursuant to Section 906 of the Sarbanes-Oxley Act of 2002 (18 U.S.C. 1350)