Denis served six years in the U.S. Army, before becoming a member of Local 355 in 1979 while employed as a driver at United Parcel Service. In 1982, he was elected by his coworkers as shop steward.
He was Sargeant-at-Arms for Local 355 in 1983, and Recording Secretary for the local in 1988. In 1991 Denis became a local business agent. He was subsequently elected president and principal officer in 1994, a leadership position he has held since.