Mathew was born and raised in Philadelphia, and was on the varsity track team of Olney High School, but graduated in 1986 from Scotland School for Veterans’ Children in Scotland, Pennsylvania, the last remaining school for children of military veterans in the United States.

In 1994, St. Patrick moved to Los Angeles to begin his acting career. In Los Angeles, he first worked as a personal trainer then in 1996 joined the West Coast Theater Ensemble.

Prior to being cast in 2001 as Keith Charles in the HBO series, Six Feet Under, St. Patrick was cast as Detective Marcus Taggert on General Hospital (1997) and as Adrian Sword on All My Children (1998–2000). He was the first voice of Skulker (2004) on Danny Phantom and starred as the principal detective in the Fox fall drama series, Reunion (2005). Additionally, he played an FBI agent in the film War (2007) and Seth in the science fiction horror film Alien Raiders (2008), and he appeared on the Eminem album Relapse (2009), in the skit “Mr. Mathers”, and in the pilot of the TV series NCIS: Los Angeles. He had a recurring role during the final season of the award-winning FX drama Sons of Anarchy, as Moses Cartwright.