Main Street Capital Corporation – Underwriting Agreements Watch

from 8-K ~10 pages Exhibit 5.1 March 31, 2026 Main Street Capital Corporation 1300 Post Oak Boulevard, Suite 800 Houston, Tx 77056 Ladies and Gentlemen: We Have Acted as Counsel to Main Street Capital Corporation, a Maryland Corporation (The “Company”), in Connection With the Preparation and Filing of a Registration Statement on Form N-2 (File No. 333-285405) (As Amended as of the Date Hereof, the “Registration Statement”) Filed by the Company With the U.S. Securities and Exchange Commission (The “Commission”) Under the Securities Act of 1933, as Amended (The “Securities Act”), Which Became Effective Immediately Upon Its Filing on February 28, 2025, and the Final Prospectus Supplement, Dated March 27, 2026 (Including the Base Prospectus Filed Therewith, the “Prospectus”), Filed With the Commission on March 31, 2026 Pursuant to Rule 424 Under the Securities Act, Relating to the Proposed Issuance by the Company of an Additional $200,000,000 Aggregate Principal Amount of 6.95% Notes Due 2029 (The “Notes”), to Be Sold to Underwriters Pursuant to an Underwriting Agreement, Dated as of March 27, 2026, Which Is Substantially in the Form Filed as Exhibit 1.1 to the Company’s Current Report on Form 8-K Filed With the Commission on March 31, 2026 (The “Underwriting Agreement”). All of the Notes Are to Be Sold by the Company as Described in the Registration Statement and Related Prospectus. This Opinion Letter Is Being Furnished to the Company in Accordance With the Requirements of Item 25 of Form N-2 Under the Securities Act, and We Express No Opinion Herein as to Any Matter Other Than as to the Legality of the Notes. the Notes Will Be Issued Pursuant to the Indenture, Incorporated by Reference as an Exhibit to the Registration Statement, Entered Into Between the Company and the Bank of New York Mellon Trust Company, N.A., as Trustee (The “Trustee”), on April 2, 2013, as Supplemented

from 8-K 34 pages Main Street Capital Corporation (A Maryland Corporation) $350,000,000 5.400% Notes Due 2028 Underwriting Agreement

from 8-K 38 pages Main Street Capital Corporation Up to 20,000,000 Shares of Common Stock (Par Value $0.01 Per Share) Equity Distribution Agreement

from 8-K 41 pages Main Street Capital Corporation (A Maryland Corporation) $100,000,000 6.500% Notes Due 2027 Underwriting Agreement

from 8-K 41 pages Main Street Capital Corporation (A Maryland Corporation) $300,000,000 6.500% Notes Due 2027 Underwriting Agreement

from 8-K 35 pages Main Street Capital Corporation (A Maryland Corporation) $350,000,000 6.950% Notes Due 2029 Underwriting Agreement

from 8-K 36 pages Main Street Capital Corporation 1,170,000 Shares of Common Stock Underwriting Agreement

from 8-K 33 pages Main Street Capital Corporation Up to 15,000,000 Shares of Common Stock (Par Value $0.01 Per Share) Equity Distribution Agreement

from 8-K 32 pages ​ Main Street Capital Corporation (A Maryland Corporation) $200,000,000 3.000% Notes Due 2026

from 8-K 32 pages Main Street Capital Corporation (A Maryland Corporation) $300,000,000 3.000% Notes Due 2026 Underwriting Agreement January 12, 2021

from 8-K 33 pages Main Street Capital Corporation (A Maryland Corporation) $125,000,000 5.200% Notes Due 2024 Underwriting Agreement July 23, 2020

from 8-K 33 pages Main Street Capital Corporation (A Maryland Corporation) $75,000,000 5.200% Notes Due 2024 Underwriting Agreement December 2, 2019

from 8-K 32 pages Main Street Capital Corporation Up to 9,500,000 Shares of Common Stock (Par Value $0.01 Per Share) Equity Distribution Agreement