Exhibit 5.1

July 14, 2015

MacroGenics, Inc.

9640 Medical Center Drive

Rockville, Maryland 20850

Ladies and Gentlemen:

This opinion is being furnished to you in connection with (i) the Registration Statement on Form S-3 (File No. 333-200092) (the “Registration Statement”) filed by MacroGenics, Inc., a Delaware corporation (the “Company”), with the Securities and Exchange Commission (the “Commission”) for the purpose of registering with the Commission under the Securities Act of 1933, as amended (the “Securities Act”), among other things, shares of the Company’s common stock, $0.01 par value per share (the “Common Stock”), which may be issued from time to time on a delayed or continuous basis pursuant to Rule 415 under the Securities Act for an indeterminate initial offering price, up to an aggregate amount of $150,000,000, as set forth in the Registration Statement and the prospectus contained therein (the “Base Prospectus”) and (ii) the prospectus supplement, dated July 14, 2015 (the “Prospectus Supplement” and, together with the Base Prospectus, the “Prospectus”) relating to the issuance and sale pursuant to the Registration Statement of up to 4,053,750 shares of Common Stock (the “Shares”), including up to 528,750 Shares issuable upon exercise of an option to purchase additional shares granted by the Company.

The Shares are to be issued and sold by the Company pursuant to an underwriting agreement dated July 14, 2015 (the “Underwriting Agreement”) by and among the Company, Morgan Stanley & Co. LLC, Citigroup Global Markets Inc. and Leerink Partners LLC, as representatives of the several underwriters named in the Underwriting Agreement, the form of which is being filed with the Commission as Exhibit 1.1 to the Company’s Current Report on Form 8-K, filed on the date hereof.

July 14, 2015

Page 2

We are acting as counsel for the Company in connection with the issue and sale of the Shares. We have examined signed copies of the Registration Statement as filed with the Commission. We have also examined and relied upon the Underwriting Agreement, minutes of meetings and actions of the stockholders and the Board of Directors of the Company as provided to us by the Company, stock record books of the Company as provided to us by the Company, the Certificate of Incorporation and Bylaws of the Company, each as restated and/or amended to date, and such other documents as we have deemed necessary for purposes of rendering the opinions hereinafter set forth.

In our examination of the foregoing documents, we have assumed the genuineness of all signatures, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as copies, the authenticity of the originals of such latter documents and the legal competence of all signatories to such documents.

We express no opinion herein as to the laws of any state or jurisdiction other than the General Corporation Law of the State of Delaware.

Based upon and subject to the foregoing, we are of the opinion that the Shares have been duly authorized for issuance and, when the Shares are issued and paid for in accordance with the terms and conditions of the Underwriting Agreement, the Shares will be validly issued, fully paid and nonassessable.

Please note that we are opining only as to the matters expressly set forth herein, and no opinion should be inferred as to any other matters. This opinion is based upon currently existing statutes, rules, regulations and judicial decisions, and we disclaim any obligation to advise you of any change in any of these sources of law or subsequent legal or factual developments which might affect any matters or opinions set forth herein.

We hereby consent to the filing of this opinion with the Commission, or the incorporation thereof by reference, as an exhibit to the Registration Statement in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act and to the use of our name therein and in the related Prospectus under the caption “Legal Matters.” In giving such consent, we do not hereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission.