Exhibit 5.1

McGuireWoods LLP

One James Center

Richmond, Virginia 23219

May 31, 2011

Markel Corporation

4521 Highwoods Parkway

Glen Allen, Virginia 23060

Ladies and Gentlemen:

We have advised Markel Corporation, a Virginia corporation (the “Company”), in connection with (i) the Registration Statement on Form S-3 (File No. 333-156265) (the “Registration Statement”), filed by the Company with the Securities and Exchange Commission for the purpose of registering under the Securities Act of 1933, as amended (the “Act”), the Company’s debt securities, among other things, to be offered from time to time by the Company on terms to be determined at the time of the offering, and (ii) the issuance by the Company of its 5.35% Senior Notes due 2021 (the “Notes”) as described in the Company’s Prospectus, dated December 18, 2008, which is a part of the Registration Statement, and Prospectus Supplement, dated May 25, 2011 (the “Prospectus Supplement”). The Notes are being issued under the Indenture dated as of June 5, 2001, between the Company and The Bank of New York Mellon (as successor to The Chase Manhattan Bank), as trustee (the “Trustee”), to be supplemented by a Sixth Supplemental Indenture dated as of June 1, 2011 (collectively the “Indenture”), the resolutions of the Board of Directors of the Company dated November 20, 2008 and the Approval of Authorized Officers dated May 25, 2011, and are being offered to the public in accordance with an Underwriting Agreement and related Pricing Agreement, dated May 25, 2011, among the Company and the Underwriters named therein. Capitalized terms used and not defined herein shall have the meanings assigned to them in the Registration Statement or the Indenture.

We have examined such corporate records, certificates and other documents, and reviewed such questions of law, as we have considered necessary or appropriate for the purpose of this opinion.

On the basis of such examination and review, we advise you that, in our opinion, when the Notes have been duly issued and sold in the manner contemplated by the Registration Statement and the Prospectus Supplement, and assuming due authentication thereof by the Trustee or the Authenticating Agent in accordance with the provisions of the Indenture, as amended and supplemented, the Notes will constitute valid and binding obligations of the Company.

Markel Corporation

May 31, 2011

Page 2

We hereby consent to the filing of this opinion as an exhibit to the Company’s Current Report on Form 8-K and the incorporation of this opinion by reference in the Registration Statement and to references to us under the heading “Legal Matters” in the Registration Statement and under the heading “Validity of Notes” in the Prospectus Supplement relating to the Notes. We do not admit by giving this consent that we are in the category of persons whose consent is required under Section 7 of the Act.