In 1990, Mahonen was chosen for the role of Gus Pike for the CBC Television program Road to Avonlea.
In 1992, he starred as a jazz trumpet player with Billy Dee Williams in the film Giant Steps.
He then played Lee Colgan in the CBC miniseries Conspiracy of Silence, directed by Francis Mankiewicz.
He played the character of Paul, a young man dying of the AIDS virus after contracting it from a blood transfusion a few years earlier.
Mahonen also guest-starred in numerous American and Canadian television projects, including an episode of Star Trek: Voyager entitled "Nemesis", in which he played a humanoid named "Brone."