Théodore played major junior in the Quebec Major Junior Hockey League (QMJHL), where he won a President's Cup as QMJHL champions and competed in the Memorial Cup with the Hull Olympiques in 1995. He won both the Ford Cup as the top defensive player and Guy Lafleur Trophy as playoff MVP in 1995 and is a two-time QMJHL Second Team All-Star. Drafted 44th overall by the Canadiens in 1994, Théodore played eight seasons in Montreal, where he won the Vezina and Hart trophies, both in 2002. In 2006, he was traded to the Colorado Avalanche, where he played two full seasons. Théodore also played two seasons for the Washington Capitals. Internationally, Théodore won a gold medal with Canada at the 1996 World Junior Championships, where he was named the tournament's best goaltender. He also started for Canada at the 2001 World Championships and was a backup for the 2004 World Cup.