A native of Detroit, Mallon is primarily a stage actor, having performed on both sides of the Atlantic. He won acclaim for his one-man show, Secrets of the Celtic Heart, directed by fellow Detroiter Ellen Burstyn, which he performed in Britain and Ireland in the early 1990s. He has served as the director of New York City's Irish Arts Center and starred in Brian Friel's plays, Philadelphia Here I Come! and Freedom of the City.
In the fall of 2005, he finished performing in Mark Jenkins' play, Playing Burton (as the late Welsh actor Richard Burton) at the Edinburgh Festival Fringe, having played New York, Los Angeles and Cardiff.
Mallon has also made a handful of film appearances. He portrayed Union Army General Winfield Scott Hancock in Ronald F. Maxwell's two Civil War films, Gods and Generals and Gettysburg. Other film roles include Some Mother's Son, The Informant and Gangs of New York.