[4] Mahon had acted while at high school, and at university he met playwright Jason Miller, who encouraged him to audition for stage plays at Marywood College.
After two years, he decided to try acting as a profession, appearing in off-Broadway productions while working as a taxi driver and waiter.
[5] In 1971, Mahon was nominated for a New York Drama Critics Award in the category Best Actor, for his performance in the play Nobody Hears a Broken Drum.
[3] Mahon guest-starred in television programs including Frasier, Cagney & Lacey, Studio 60 on the Sunset Strip (7 episodes), Just Shoot Me!, Jake and the Fatman, Steambath, Knots Landing, Generations, The X-Files, Hill Street Blues, St.
[3][1][2] In 1995, Mahon played the role of the chairman of the Joint Chiefs of Staff in the film The American President.