He portrayed Jim Harper in Aaron Sorkin's drama series The Newsroom, starred in the HBO mini-series Olive Kitteridge, and played Emmett DeWitt in 10 Cloverfield Lane.

[10] Gallagher starred in productions of the dramas, Port Authority, by Conor McPherson and Farragut North by Beau Willimon.

Gallagher starred in American Idiot, a musical based on the Green Day album of the same name, along with fellow Spring Awakening alumni Brian Charles Johnson and Gerard Canonico.

Gallagher departed American Idiot on February 27, 2011, and was succeeded by Van Hughes[15] The show's run ended on April 24, 2011.

After leaving American Idiot, Gallagher was featured in the Broadway production of The Royal Court Theatre's Jerusalem.

The production was directed by Ian Rickson, and opened on April 21, 2011, at the Music Box Theatre (239 West 45th Street).

[17] He starred as Edmund Tyrone in Roundabout Theatre Company's revival of Eugene O'Neill's Long Day's Journey into Night, alongside Jessica Lange and Gabriel Byrne.

[19] Gallagher has appeared in several television programs, including The West Wing, Law & Order: Special Victims Unit, NYPD Blue, Ed, Love Monkey, and Hallmark Hall of Fame's film The Flamingo Rising.

Gallagher's film credits include Pieces of April, Woody Allen's Whatever Works, Jonah Hex, The Heart Machine, Kenneth Lonergan's Margaret, and a lead role in Short Term 12 opposite Brie Larson.

Short Term 12's director, Destin Daniel Cretton described Gallagher as having an innate quality, that the actor "feels like a big brother anyone would want to have.