Exhibit 5.1

Hogan Lovells US LLP

Harbor East

100 International Drive, Suite 2000

T +1 410 659 2700

F +1 410 659 2701

www.hoganlovells.com

July 8, 2014

Board of Directors

Capstead Mortgage Corporation

8401 N. Central Expressway

Suite 800

Dallas, TX 75255-4411

2014 Flexible Incentive Plan

Ladies and Gentlemen:

We are acting as counsel to Capstead Mortgage Corporation, a Maryland corporation (the “Company”), in connection with its registration statement on Form S-8 (the “Registration Statement”), filed with the Securities and Exchange Commission under the Securities Act of 1933, as amended (the “Act”), relating to the proposed offering of 5,000,000 newly issued shares of the common stock, par value $0.01 per share (the “Common Stock”) of the Company (the “Shares”), all of which shares are issuable pursuant to the Company’s Amended 2014 Flexible Incentive Plan (the “Plan”). This opinion letter is furnished to you at your request to enable you to fulfill the requirements of Item 601(b)(5) of Regulation S-K, 17 C.F.R. § 229.601(b)(5), in connection with the Registration Statement.

For purposes of this opinion letter, we have examined copies of such agreements, instruments and documents as we have deemed an appropriate basis on which to render the opinions hereinafter expressed. In our examination of the aforesaid documents, we have assumed the genuineness of all signatures, the legal capacity of all natural persons, the accuracy and completeness of all documents submitted to us, the authenticity of all original documents, and the conformity to authentic original documents of all documents submitted to us as copies (including telecopies). We also have assumed that the Shares will not be issued in violation of the ownership limit contained in the Company’s charter. As to all matters of fact, we have relied on the representations and statements of fact made in the documents so reviewed, and we have not independently established the facts so relied on. This opinion letter is given, and all statements herein are made, in the context of the foregoing.

This opinion letter is based as to matters of law solely on the Maryland General Corporation Law, as amended. We express no opinion herein as to any other laws, statutes, ordinances, rules, or regulations.

July 8, 2014 Page 2

Based upon, subject to and limited by the foregoing, we are of the opinion that, following (i) effectiveness of the Registration Statement, (ii) issuance of the Shares pursuant to the terms of the Plan and any applicable award agreements entered into thereunder, and (iii) receipt by the Company of the consideration for the Shares specified in the applicable resolutions of the Board of Directors and in the Plan and any applicable award agreements, the Shares will be validly issued, fully paid, and nonassessable.

This opinion letter has been prepared for use in connection with the Registration Statement. We assume no obligation to advise you of any changes in the foregoing subsequent to the effective date of the Registration Statement.

We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the Registration Statement. In giving this consent, we do not thereby admit that we are an “expert” within the meaning of the Securities Act of 1933, as amended.

Very truly yours,

/s/ HOGAN LOVELLS US LLP

HOGAN LOVELLS US LLP