He played in the National Hockey League (NHL) for the Montreal Canadiens, Colorado Avalanche, Washington Capitals, Minnesota Wild, and Florida Panthers.

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.

Internationally, Théodore won a gold medal with Canada at the 1996 World Junior Championships, where he was named the tournament's best goaltender.

Théodore went on to lead the Olympiques to the President's Cup as QMJHL champions, winning the Guy Lafleur Trophy as playoff MVP.

He made his Stanley Cup playoffs debut in 1997, winning a 4–3 triple overtime game against the New Jersey Devils, making 56 saves.

Théodore emerged as a world-class goaltender in 2001–02, when he turned in a Vezina- and Hart Memorial Trophy-winning performance with a 30–24–10 record, 2.11 GAA and .931 save percentage.

[5] He led the Canadiens into the playoffs as the eighth and final seed in the Eastern Conference, and was a pivotal factor in upsetting the top-ranked Boston Bruins in the first round.

They upset the Boston Bruins for the second time in three years in a seven-game opening series, before being eliminated by the top-seeded, eventual Stanley Cup champion Tampa Bay Lightning in four.

When NHL play resumed in 2005–06, it was revealed on February 9, 2006, he had failed a random drug test conducted prior to the 2006 Winter Olympics.

The failed test was later revealed to be caused by a prescription hair loss medication Propecia, which Théodore had been taking legally for eight years.

[7] In addition to the drug controversy, Théodore's play with the Canadiens was marked by a significant drop and he was being outperformed by backup Cristobal Huet.

[8] At the time of the trade, Théodore was on the injured reserve; he strained his Achilles tendon after slipping on the winter ice outside his home.

Théodore's play did not see much improvement the following season, in 2006–07, as he lost the starting role to Budaj with a 13–15–1 record, 3.26 GAA and .891 save percentage.

He was able to lead the Avalanche to the second round of the 2008 Stanley Cup Playoffs, though they would be dominated by the Detroit Red Wings in a four-game sweep.

[9] He replaced long-time Capitals starter Olaf Kölzig and the previous season's acquisition (as well as former Canadiens teammate) Cristobal Huet, both having departed in free agency.

However, after allowing four goals in a Game 1 loss to the New York Rangers in the opening round, he was pulled in favour of backup Semyon Varlamov.

[12] In need of a netminder the Wild signed Théodore to a one-year, $1.1 million contract to serve as backup to Niklas Bäckström.

Despite having home ice advantage in the first round, the Panthers would lose Game 7 to the New Jersey Devils in double overtime, 3–2, with Théodore stopping 33 of 36 shots.