He was born in Manhattan but raised in Armonk. After attended Byram Hills High School, he graduated from Tufts University, where he was active in theater.
Gallagher is perhaps best known for his role of Sandy Cohen in "The O.C.," but he's had roles in numerous TV shows and films, including "Covert Affairs," "Californication," "American Beauty" and "While You Were Sleeping."
Tufts University honored Gallagher with its "Light on the Hill" award in 2007, given to alumni "to recognize their leadership within their fields and for serving as outstanding role models," the university alumni website notes.
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