Muldoon was born in Los Angeles, California, the son of Deanna, a homemaker, and william Patrick Muldoon II, a personal injury Lawyer. He is of Irish descent on his father's side and of Croatian descent on his mother's side. He attended Loyola High School, a Jesuit school. Muldoon graduated in 1991 from the University of Southern California, where he was a member of the Sigma Chi fraternity and played tight end on the USC Trojans football team.