An undrafted player, St. Louis played a total of 1,134 games, scoring 391 goals and 1,033 points in an NHL career that began with the Calgary Flames in 1998 and ended with the New York Rangers in 2015.

[5] He tied friend and teammate Éric Perrin for the NCAA scoring lead and the pair, along with goaltender Tim Thomas, led the Catamounts to the first ECAC hockey championship in school history.

[6] Named an all-star at the 1996 NCAA Division I Men's Ice Hockey Tournament,[5] St. Louis scored the winning goal in a 2–1 victory over Lake Superior State to reach the Frozen Four.

[10] A finalist for the Hobey Baker Award for the first of two consecutive seasons,[11] St. Louis earned interest from National Hockey League (NHL) teams in the summer of 1996.

[5] St. Louis finished as Vermont's all-time leading scorer with 267 points, a record he continues to hold as of 2023, as well as his school mark of 176 assists.

[8] He won the J. Edward Donnelly Award as the top male senior athlete at the University of Vermont in 1997 and was named to the ECAC's all-decade team of the 1990s.

He was assigned to their then-American Hockey League (AHL) affiliate, the Saint John Flames, where he scored 26 points in 25 regular season games.

[7] St. Louis earned a spot on the Calgary roster to begin the 1998–99 season and made his NHL debut on October 9, 1998, against the San Jose Sharks.

[15] He began the season playing with Calgary's top forward, Theoren Fleury, but was quickly demoted to the fourth line, and often sat out of the lineup.

[7] He appeared in only 13 games in Calgary, spending the majority of the season in Saint John where he led the AHL squad with 28 goals and 62 points.

The Flames exposed him in the 2000 NHL Expansion Draft, but after he went unselected, the team bought out his contract and made him an unrestricted free agent.

[16] While leading the team with 16 goals midway through the 2001–02 season, St. Louis suffered a broken leg following an awkward check by Josef Melichar of the Pittsburgh Penguins on January 23, 2002.

[16] St. Louis exceeded his previous season in 2003–04, capturing the Art Ross Trophy as the NHL's leading scorer with 94 points (38 goals, 56 assists).

St. Louis was only the eighth player in NHL history to win the Art Ross and Hart Trophies and the Stanley Cup in the same season, and the first to do so since Wayne Gretzky in 1986–87.

[16] Finishing with only 12 penalty minutes on the season, he was voted the recipient of the Lady Byng Memorial Trophy as the NHL's most gentlemanly player.

[26] He won the Lady Byng Trophy for a second time in 2010–11,[27] the culmination of a season in which St. Louis made his sixth All-Star Game appearance, was named to the second All-Star team for the third time and finished second in league scoring with 99 points (31 goals, 68 assists) only behind Daniel Sedin of the Vancouver Canucks, who finished with 104 points.

Lightning general manager Steve Yzerman praised St. Louis' importance to the organization when announcing the signing: "Marty means so much to this franchise, both on and off the ice.

The streak came to an end at 499 consecutive games played in early December 2011 when he was struck in the face by a shot from teammate Dominic Moore during practice.

St. Louis set a career high and tied a Lightning franchise record by scoring four goals in a January 18, 2014, game against the San Jose Sharks.

[40] The no movement clause in his contract gave St. Louis control over where he could be moved and he reportedly consented to only being traded to the New York Rangers, a request he first made in 2009.

[43] The death of his mother, France, of a heart attack on May 8 left St. Louis' participation in Games 5 and 6 of New York's second-round playoff series against the Pittsburgh Penguins in doubt.

[39] St. Louis led the team with 8 goals in the playoffs and finished second with 15 points,[45] but the Rangers lost the final to the Los Angeles Kings in five games.

[47] Two days later, St. Louis recorded a goal and an assist against the Philadelphia Flyers to reach 1,000 points in his career, becoming the sixth undrafted player in NHL history to do so.

[59] On February 6, 2014, he was selected to replace injured former Lightning teammate Steven Stamkos for representation at the 2014 Winter Olympics[60] and played in five of Canada's six games.

On January 21, 2019, he became the special teams consultant for the Columbus Blue Jackets, working alongside his former Tampa Bay Lightning head coach, John Tortorella.

[70] On April 17, 2024, the Canadiens exercised a two-year option on St. Louis' contract, effectively making his tenure as head coach until at least the conclusion of the 2026–27 season.

[74] Hockey Hall of Fame coach Scotty Bowman credited St. Louis' desire to succeed as being his primary attribute: "His long suit is his passion.

[76] An offensive minded player throughout his amateur and minor professional career, St. Louis was forced into a checking-line role by the Calgary Flames.

"[7] St. Louis met his wife Heather Anne Caragol at the University of Vermont,[75] where he graduated in 1997 with a degree in small business management.

[78] St. Louis runs an annual summer hockey camp in the nearby community of Stamford for young players in the region.