Théodore became a full-time NHLer in 1999–2000, sharing starts with Jeff Hackett. In his first full NHL season, Théodore posted a 12–13–2 record with a 2.10 GAA and .919 save percentage, along with five shutouts. He assumed the starting role over Hackett the following season in 2000–01 and went 20–29–5 in 59 games. During a game on January 2, 2001, Théodore became the sixth goaltender to directly score a goal when he attempted to clear the puck from the defensive zone against the New York Islanders and scored into the empty net, which was vacated by John Vanbiesbrouck for the extra attacker. He became the first NHL goalie to directly score a goal and record a shutout in the same game, as the Canadiens defeated the Islanders 3–0. But he was the second goaltender to be credited with a goal and a shutout in the same game, after Damian Rhodes, who was credited with a goal in a 6–0 win on January 2, 1999.
Théodore made his debut for Canada's men's team in the 2001 World Championship. He recorded two shutouts and a 1.63 GAA, but Canada was defeated in the quarter-finals by the United States. In 2004, he played backup for Team Canada at the World Cup, seeing Canada defeat Finland in the final to capture the championship.