Born in Kilmarnock, Scotland, he moved with his family to Canada in 1964. He is an alumnus of Toronto's famous Second City comedy troupe, where he co-wrote and starred in three productions as well as directed three seasons of their National Touring Company. After nine hugely successful years as a regular on the British Channel 4 hit improvisation series Whose Line is it Anyway, he became a regular on the American version hosted by Drew Carey which aired on ABC. Already one of the most highly visible actors in North American television, joined This Hour Has 22 Minutes on CBC TV in 2001 through 2003. However he is no longer part of the show due to movies and other TV commitments.
Mochrie married Debra McGrath in 1989, and they have a son, Luke.