Born in Chicago, Illinois, Kerwin won the Theatre World Award in 1988 for the off-Broadway play Emily. His Broadway theatre credits include the 1997 revival of The Little Foxes and the Elaine May comedy After the Night and the Music in 2005. That same year he starred in Edward Albee's The Goat or Who is Sylvia? at the Mark Taper Forum. He also played opposite Kathy Baker in the South Coast Repertory production of The Man from Nebraska in 2006. His most recent stage appearance was in the Broadway production August: Osage County.