[10] Varney became known for spearheading the FTC's examination of privacy and commerce,[11] and for promoting innovation market theory analysis in the fields of information technology and biotechnology.

[13] Varney was a partner at the Washington, D.C. law firm Hogan & Hartson, where she chaired the Internet practice group,[6] and was registered as a lobbyist.

[14][15] As a lawyer, Varney represented and advised companies on matters such as antitrust, privacy, business planning corporate governance, intellectual property, and general liability issues.

[18] Varney was nominated for the position of Assistant Attorney General for the Antitrust Division of the Department of Justice in February 2009, and confirmed by the Senate on April 20, 2009.

[26] There, Varney has advised clients on mergers across various industries, including acquisitions of Time Warner by ATT, Virgin Atlantic by Delta, Talenti by Unilever, Pinnacle by Conagra, and Heinz by Kraft[27] Varney joined the faculty of Columbia Law School as a lecturer in law in 2017 and teaches the course Antitrust in Action, alongside her Cravath colleague David R.

[28] Varney is recognized by The National Law Journal as one of the "100 Most Influential Lawyers in America", and as among the 50 "Governance, Risk & Compliance Trailblazers & Pioneers".

[29] During the 2020 Democratic Party presidential primaries, Varney made the maximum allowable campaign contributions to candidates Pete Buttigieg[30] and Joe Biden.

"[32] Former FTC Chairman Robert Pitofsky has credited Varney as "the leading force in getting the agency active on the online privacy front.

[42] As a Commissioner at the FTC, Varney was outspoken about monopolies in innovation markets and about the possibility that vertical mergers create unfair barriers to entry in networked industries.

[46] In her first public comments as an Assistant Attorney General, Varney criticized the guidelines for "effectively straightjacket[ing] antitrust enforcers and courts from redressing monopolistic abuses, thereby allowing all but the most bold and predatory conduct to go unpunished and undeterred.

[48] Varney opened inquiries into the financial services and wireless phone industries, and began probing the settlement between Google and the Association of American Publishers.

As an FTC Commissioner, Varney stated, "there is much more to be done by way of fostering communication and cooperation between enforcement authorities," and promoted adherence to international antitrust guidelines.