from 2003 to 2007, and recurring roles in television such as Deputy Chief William Dodds on Law & Order: Special Victims Unit, Stacey Koons on the Showtime comedy-drama Californication, Nick on the Netflix series Grace & Frankie, and Director of Clandestine Services (DCS) Arthur Campbell on Covert Affairs.

His mother, Mary Ann (née O'Shea), was a bacteriologist, and his father, Thomas Francis Gallagher, Jr., was an advertising executive.

Gallagher graduated from Tufts University, where he was active in theater, appearing in such shows as Stephen Sondheim's Company and singing with the all-male a cappella group the Beelzebubs.

From 2003 to 2007, Gallagher starred as Sandy Cohen, a Jewish public defender and corporate lawyer, on the Fox television show The O.C.

This includes a studio recording of his performance of "Don't Give Up On Me" (originally by Solomon Burke), which was featured in an episode of The O.C.

[8] In 2020 Gallagher played Mitch Clarke, who had progressive supranuclear palsy, in a regular role in the first season of Zoey's Extraordinary Playlist.

In 2024, Gallagher is set to return to Broadway, in a new play based on the life of Delia Ephron, titled Left on Tenth.