Ashmore was born in Richmond, British Columbia. His identical twin brother, Aaron Ashmore, is also an actor. Aaron and Shawn have played twins in several movies, but have also pursued roles independently. According to Aaron, Shawn often gets cast as the nice guy while Aaron himself is cast as the bully. Ashmore's most famous role may be as Iceman in X-Men and its sequels X2 and X-Men: The Last Stand. Ashmore starred as Cadet Major Brad Rigby in Cadet Kelly, a Disney Channel original film and he also guest-starred as Eric Summers in two episodes of Smallville.