Having studied at the Juilliard School under the supervision of John Houseman, Gallagher began her career on stage, and has appeared in several Broadway theatre productions, winning a Theatre World Award for her role in A Few Good Men.
Belajar di Juilliard School di bawah naungan John Houseman, Gallagher memulai kariernya di panggung, dan tampil dalam beberapa produksi teater Broadway, memenangkan sebuah Theatre World Award untuk perannya dalam A Few Good Men.