Exhibit 5.1

May 18, 2020

Avantor, Inc.

Radnor Corporate Center

Building One, Suite 200

100 Matsonford Road

Radnor, PA 19087

Ladies and Gentlemen:

We have acted as counsel to Avantor, Inc., a Delaware corporation (the “Company”), in connection with the Registration Statement on Form S-1 (the “Registration Statement”) filed by the Company with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), relating to the sale by the selling stockholders referred to in the Registration Statement (the “Selling Stockholders”) of an aggregate of up to 51,750,000 shares of Common Stock, par value $0.01 per share, of the Company (the “Common Stock”) (together with any additional shares of such Common Stock that may be sold by the Selling Stockholders pursuant to Rule 462(b) (as prescribed by the Commission pursuant to the Securities Act) in connection with the offering described in the Registration Statement, the “Shares”).

We have examined the Registration Statement and the Second Amended and Restated Certificate of Incorporation of the Company (the “Amended Charter”), which has been filed with the Commission as an exhibit to the Registration Statement. In addition, we have examined, and have relied as to matters of fact upon, originals, or duplicates or certified or conformed copies, of such records, agreements, documents and other instruments and such certificates or comparable documents of public officials and of officers and representatives of the Company and have made such other investigations as we have deemed relevant and necessary in connection with the opinions hereinafter set forth.

In rendering the opinions set forth below, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as duplicates or certified or conformed copies and the authenticity of the originals of such latter documents.

Based upon the foregoing, and subject to the qualifications, assumptions and limitations stated herein, we are of the opinion that the Shares have been validly issued and are fully paid and nonassessable.

We do not express any opinion herein concerning any law other than the law of the State of New York and the Delaware General Corporation Law.

We hereby consent to the filing of this opinion letter as Exhibit 5.1 to the Registration Statement and to the use of our name under the caption “Legal Matters” in the prospectus included in the Registration Statement.

Very truly yours,

/s/ Simpson Thacher & Bartlett LLP

SIMPSON THACHER & BARTLETT LLP