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Cahill was born in New York City, New York. He is the middle of three children with one older and one younger sister. He is of Irish descent from his father, a stockbroker, and of Italian descent from his mother, an elementary school teacher.
Cahill graduated from Byram Hills High School in Armonk, New York in 1996. He attended Skidmore College in Saratoga Springs, New York and the Atlantic Theater Acting School, part of the Tisch School of the Arts at NYU.
In July 2009,Cahill married his longtime girlfriend Nikki Uberti in Los Angeles, California. Their son Henry was born in 2009.