Sean grew up in the Chicago suburb of Glen Ellyn, Illinois. His father, Ron, a lithographer, left him and his four siblings when Sean was a young child. Sean says he hasn't seen his father in more than 10 years. His mother, Mary, works at a food bank. Sean attended Illinois State University where he studied music and theatre. He supported himself through his college years by playing piano for voice lessons and working in community theater. He played a stint with the Second City comedy troupe and then became a musical director for a local theater. He grew tired of that after a while and in the spring of 1995, moved to LA. He got a "hellish" job at a restaurant. His first big break was landing a national tour with Kenny Rogers for a Christmas show. In 1997, he got a series of commercials that gained him attention and led to his eventual role on Will & Grace (1998).