Prior to acting, he had worked as a doorman and a stand-up Comedian. While working at the Boston Office of Cultural Affairs, Chapman was discovered by the late Director, Ted Demme, and was cast as Mickey Pat in Monument Ave. (1998). Other notable film roles of his include The Cider House Rules, 21 Grams, In Good Company, an Italian mobster in Boondock Saints (1999), and Fire Lt. Frank McKinney in Ladder 49 (2004). Chapman starred as Irish Mob boss, Freddie Cork, for three seasons in the Showtime original series Brotherhood. Chapman played a first responder assisting Jack Bauer (Kiefer Sutherland) in season 1 episode 24 of the TV series 24 (2001) and Kevin Mitchell in season 3 of 24 (2003). and a CIA operative (O'Leary) in the independent comedy Black Dynamite.

In 2006, Chapman appeared as an officer in the episode "Lessons Learned" (Season 2, Episode 10) of Criminal Minds. In 2010, he played Bunny in Tony Scott's action film Unstoppable. He also was a guest star in the series Lost. He appears in the film Bad Country (2013), directed by Chris Brinker. From 2011 to 2016, he portrayed Detective Lionel Fusco on the CBS show Person of Interest.

He is a Massachusetts native and, as of 2013, currently lives there with his wife and two children.