Ashmore's most famous role may be as Iceman in X-Men and its sequels X2 and X-Men: The Last Stand. While many of the minor characters were portrayed by different actors, Ashmore was kept for all the movies since he was major in the first movie, unlike Kitty Pryde who only gets 2 seconds screen time in the first and 10 in the second.
Ashmore starred as Major Brad Griby in Cadet Kelly, a Disney Channel original film which aired in 2002 and he also guest-starred as Eric Summers in two episodes of Smallville, a series on which his brother later got the role of Jimmy Olsen. Ashmore also had starring roles on Animorphs as Jake Berenson and In a Heartbeat as Tyler Connell.
In December of 2004, Ashmore was cast in the lead role in the SciFi Channel mini series Legend of Earthsea, based on the novels by Ursula K. Le Guin. He played the role of Ged, a young wizard-in-training, who takes advice from Ogion (Danny Glover) and falls in love with Tenar (Kristin Kreuk), the protege of the High Priestess of the Tombs of Atuan (Isabella Rossellini). In 2005 Ashmore starred in a CTV TV movie about Terry Fox's historic run across Canada, which aired in September.