McSweeny was confirmed 95-1, with Louisiana Republican David Vitter the sole no vote.
McSweeny, who was nominated by President Barack Obama in the White House in June, was the former domestic policy advisor to Vice President Joe Biden. She joins FTC Chairwoman Edith Ramirez, a former law school classmate of the president, and Julie Brill, in the majority.
Maureen Ohlhausen and Joshua Wright are the two Republicans on the board, which works with the Justice Department to enforce anti-trust laws and investigates issues such as deceptive advertising.
Prior to McSweeny's confirmation, there was significant concern over a potential for a 2-2 tie in decisions dealing with several major upcoming mergers, including those of Kroger and Harris Teeter, Fidelity National Financial and Lender Processing Services, and funeral home companies Stewart Enterprises and Service Corp International.
In the case of a tie among commissioners, the FTC does not intervene.