Exhibit 5.1

May 13, 2020

Alector, Inc.

131 Oyster Point Boulevard, Suite 600

South San Francisco, California 94080

Re:     Alector, Inc. – “At the Market” Sale and Issuance of up to $150 million of] Shares of Common Stock

Ladies and Gentlemen:

This opinion is furnished to you in connection with the Registration Statement on Form S-3 (the “Registration Statement”), filed by Alector, Inc. (the “Company”) with the Securities and Exchange Commission on May 13, 2020 pursuant to Rule 462(e) of the Securities Act of 1933, as amended (“Securities Act”), the prospectus contained within the registration statement (the “Prospectus”) and the prospectus supplement to the Registration Statement dated May 13, 2020 (the “Prospectus Supplement”) in connection with the registration under the Securities Act of up to $150 million of shares the Company’s common stock, $0.0001 par value per share (the “Shares”), to be issued and sold by the Company. We understand that the Company has agreed to issue and sell the Shares from time to time through Morgan Stanley & Co. LLC and Goldman Sachs & Co. LLC, as sales agents (the “Sales Agents”) pursuant to an equity distribution agreement by and among the Company and the Sales Agents (the “Equity Distribution Agreement”).

We are acting as counsel for the Company in connection with the issuance and sale of the Shares by the Company. In such capacity, we have examined originals or copies, certified or otherwise identified to our satisfaction, of such documents, corporate records, certificates of public officials and other instruments as we have deemed necessary for the purposes of rendering this opinion. In our examination, we have assumed the genuineness of all signatures, the authenticity of all documents submitted to us as originals, the conformity with the originals of all documents submitted to us as copies, the authenticity of the originals of such 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 (including the statutory provisions and all applicable judicial decisions interpreting those laws) and the federal laws of the United States of America.

On the basis of the foregoing, we are of the opinion that the Shares to be issued and sold by the Company have been duly authorized and, when such Shares are issued and paid for in accordance with the terms of the Equity Distribution Agreement, will be validly issued, fully paid and nonassessable.

AUSTIN         BEIJING         BOSTON         BRUSSELS         HONG KONG         LONDON         LOS ANGELES         NEW YORK

PALO ALTO         SAN DIEGO         SAN FRANCISCO         SEATTLE         SHANGHAI         WASHINGTON, DC         WILMINGTON, DE

May 13, 2020

Page 2

We hereby consent to the filing of this opinion as an exhibit to the Company’s Current Report on Form 8-K filed on or about May 13, 2020, for incorporation by reference into the Registration Statement, and to the use of our name wherever it appears in the Registration Statement, the Prospectus, Prospectus Supplement, and in any amendment or supplement thereto. In giving such consent, we do not believe that we are “experts” within the meaning of such term as used in the Act or the rules and regulations of the Commission issued thereunder with respect to any part of the Registration Statement, including this opinion as an exhibit or otherwise.