Exhibit 5.1

May 12, 2014

American Tower Corporation

116 Huntington Avenue

Boston, Massachusetts 02116

Ladies and Gentlemen:

We have acted as special counsel to American Tower Corporation, a Delaware corporation (the “Company”), in connection with its offering pursuant to a registration statement on Form S-3 (No. 333-188812), as amended as of its most recent effective date (May 6, 2014), insofar as it relates to the Securities (as determined for purposes of Rule 430B(f)(2) under the Securities Act of 1933, as amended (the “Securities Act”)) (as so amended, but excluding the documents incorporated by reference therein, the “Registration Statement”) and the prospectus, dated May 23, 2013 (the “Base Prospectus”), as supplemented by the prospectus supplement thereto, dated May 6, 2014 (the “Prospectus Supplement” and, together with the Base Prospectus, the “Prospectus”), of 6,000,000 shares of the Company’s 5.25% Mandatory Convertible Preferred Stock, Series A, par value $0.01 per share (the “Securities”).

In arriving at the opinions expressed below, we have reviewed the following documents:

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In addition, we have reviewed the originals or copies certified or otherwise identified to our satisfaction of all such corporate records of the Company and such other documents, and we have made such investigations of law, as we have deemed appropriate as a basis for the opinions expressed below.

In rendering the opinions expressed below, we have assumed the authenticity of all documents submitted to us as originals and the conformity to the originals of all documents submitted to us as copies. In addition, we have assumed and have not verified the accuracy as to factual matters of each document we have reviewed.

Based on the foregoing, and subject to the further assumptions and qualifications set forth below, it is our opinion that:

1. The Securities have been validly issued by the Company and are fully paid and nonassessable.

2. The shares of the Company’s common stock, par value $0.01 per share (the “Common Stock”), into which the Securities are convertible have been authorized and reserved for issuance upon conversion and, upon initial issuance thereof upon conversion of the Securities in accordance with the Certificate of Designations and the terms of the Securities at conversion prices at or in excess of the par value of such shares of Common Stock, will be validly issued, fully paid and nonassessable.

Insofar as the foregoing opinions relate to the validity, binding effect or enforceability of any agreement or obligation of the Company, (x) we have assumed that the Company and each other party to such agreement or obligation has satisfied those legal requirements that are applicable to it to the extent necessary to make such agreement or obligation enforceable against it (except that no such assumption is made as to the Company regarding matters of the General Corporation Law of the State of Delaware that in our experience normally would be applicable to general business entities with respect to such agreement or obligation) and (y) such opinions are subject to applicable bankruptcy, insolvency and similar laws affecting creditors’ rights generally and to general principles of equity.

The foregoing opinions are limited to the General Corporation Law of the State of Delaware.

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We hereby consent to the use of our name in the Prospectus Supplement under the heading “Legal Matters” and in the Base Prospectus under the heading “Validity of the Securities,” as counsel for the Company that has passed on the validity of the Securities and to the filing of this opinion letter as Exhibit 5.01 to the Company’s Current Report on Form 8-K dated May 12, 2014. In giving such consent, we do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Securities and Exchange Commission thereunder.

The opinions expressed herein are rendered on and as of the date hereof, and we assume no obligation to advise you or any other person, or to make any investigations as to any legal developments or factual matters arising subsequent to the date hereof that might affect the opinions expressed herein.