Stiles was born in Seattle, Washington, the youngest of five children. His father, Sonny, was a fishing plant supervisor. Stiles lived in Vancouver, British Columbia as a teenager. At the age of seventeen, he dropped out of his high school, Richmond Secondary, to have a career in comedy. He worked in his father's fish-processing plant to earn a living. He then began performing improv with Vancouver TheatreSports League. In 1986, Stiles successfully auditioned for a berth at Toronto's renowned Second City comedy ensemble.