Denis was born to Irish immigrants Nora and John Leary. He attended Emerson College in Boston, dabbling in acting and writing. He was a founding member of Emerson's Comedy Workshop.
He worked as a stand-up comic in the twilight of the late-80's comedy boom, and achieved some notoriety for his acerbic take on American life. His comedy CD No Cure For Cancer
featured the hit novelty song, I'm An A**hole
. He also appeared on the MTV game show Remote Control
Denis began appearing in movies, including The Ref
, Demolition Man
, and Two If By Sea
. He voiced a character in the successful animated film Ice Age
, and appeared as the lead in the FX fireman hit, Rescue Me
, for which he was nominated for a Golden Globe.