Born in Ottawa, Cumyn graduated in 1984 from Queens University with Honours in Theatre and English, where he received the Lorne Greene Award for Acting. He started acting professionally in theatre, film and television right after college. His breakout role, one that earned him a Gemini Award nomination, was in the soap opera series "Paradise Falls" in which he played a radio personality wannabe star.

He has been living and working in Toronto ever since and is most proud of his contribution to the life of new Canadian theatre. Cumyn has had roles in the TV movies and series Nero Wolf; Her Best Friend's Husband and Blue Murder and in the features Sins of the Father; K 19: The Widowmaker and Owning Mahowney. Cumyn is also a regular performer on stage at London's Stratford Festival.

Recent credits include The Orphan Muses, Martin Yesterday, and Angels in America Parts I & II, for which he received a Dora Award for his portrayal as Prior Walter. Other credits include A Midsummer Night's Dream and Twelfth Night, W.W.II, Never Swim Alone, and A Secret Life.