Gallagher began her acting career in 1996, when she appeared in a small role in the film The Juror. She then went on to appear in a number of other films and television shows, including The Rage: Carrie 2 (1999), Confessions of a Teenage Drama Queen (2004), and The O.C. (2004-2007).
In 2007, Gallagher made her Broadway debut in the play Reasons to Be Pretty. She has since appeared in a number of other stage productions, including The Glass Menagerie (2010) and The Heidi Chronicles (2015).
In addition to her acting career, Gallagher is also a successful fashion model. She has appeared in campaigns for a number of major brands, including Calvin Klein, Tommy Hilfiger, and Ralph Lauren.
Gallagher is currently married to actor and photographer Zak Orth. The couple have one child together, a daughter named Mae Elizabeth.