Junior in April 2009 As a youth playing within the South Delta Minor Hockey Association, Gallagher was selected in the ninth round of the 2007
Western Hockey League (WHL)
bantam draft by the
Vancouver Giants. After a single season with the Greater Vancouver Canadians of the
British Columbia Hockey Major Midget League (BCMML), he joined the Giants for his rookie campaign in
2008–09, scoring his first WHL goal on September 27, 2008 in a 7–3 win over the
Prince George Cougars. Finishing the regular season with ten goals and 31 points over 52 games, Gallagher added three points (one goal, two assists) in 16 postseason contests, as the Giants were eliminated in the semifinals by the
Kelowna Rockets. The
following season, he improved to 81 points over 72 games, ranking second among Giants players behind
Craig Cunningham, while his 41 goals were a team high. His strong play continued into that year's playoffs, helping the Giants to the semifinals for a second consecutive year. Overall, Gallagher registered 21 points (11 goals and 10 assists) in 16 postseason games, ranking second in team-scoring and fifth across the league, as Vancouver was eliminated by the
Tri-City Americans. During the playoffs of his draft eligible season, Gallagher was selected as the Male Youth Athlete of the Year for his hometown of
Delta, British Columbia. In March 2011, Gallagher was sidelined with a head injury, sustained after receiving a hit in a game against the
Portland Winterhawks. Despite this setback, Gallagher finished first in team scoring as well as eighth amongst all WHL players with 91 points over 66 games, earning himself recognition as part of the league's
West First All-Star Team. His 44 goals were just four shy of
Evander Kane's single-season team record. Appearing in his second training camp with the Canadiens leading up to the
2011–12 season, Gallagher was signed to a three-year, entry-level contract upon return to his junior squad. In December 2011, he temporarily left the Giants for the
Canadian national junior team. On January 7, 2012, Gallagher recorded a seven-point effort in his WHL return including a
hat-trick against the Portland Winterhawks. Three days later, he assumed
team captaincy after predecessor James Henry was dealt to the
Moose Jaw Warriors. Not long after, Gallagher sustained an upper-body injury after colliding into the boards while pursuing a loose puck in a game against the Tri-City Americans. Following return to the lineup in late January, Eleven days later, he recorded two goals in a 5–3 win against the
Kamloops Blazers for his record-setting 274th point as a Giant. Near the end of the season, he was voted by a panel of WHL players, executives and broadcasters as the
most valuable player to his team in the
Tri-City Heralds annual survey and once again earned league West First All-Star Team honors. Over the course of his junior career, Gallagher had earned a reputation as a fast-skating scorer who plays with energy and aggression, making him adept in both offensive and defensive situations.
Professional In his first professional season, Gallagher was assigned to the Canadiens'
American Hockey League (AHL) affiliate, the
Hamilton Bulldogs, due to the ongoing
NHL lockout at the time. Upon a resolution, he was recalled to the Canadiens' training camp and joined the team for the shortened
2012–13 season. Gallagher scored his first NHL goal on January 27, 2013 against
Martin Brodeur in a 4–3 win over the
New Jersey Devils. After the reacquisition of
Michael Ryder by the Canadiens in February 2013, Gallagher switched from jersey number 73 to 11 to accommodate the veteran forward, who had worn 73 throughout his NHL career. It was later revealed that the foregoing had offered Gallagher a
Rolex watch in return of the number 73, however, this promise was never fulfilled. He ended his rookie campaign with a total of 15 goals and 13 assists in 44 games, establishing himself as a key member of Montreal's offence, often playing on the Canadiens' first line. On May 6, 2013, Gallagher received nomination for the
Calder Memorial Trophy as the NHL's Rookie of the Year, and placed second in voting. On November 29, 2014, the Canadiens agreed to terms with Gallagher on a six-year contract extension. . On September 18, 2015, the team announced Gallagher as one of their newly appointed alternate captains along with
P. K. Subban,
Tomáš Plekanec, and
Andrei Markov. During the course of the
2015–16 season, Gallagher broke two fingers after blocking a shot from
Johnny Boychuk and would require corrective surgery. He returned to the Canadiens lineup for the
2016 NHL Winter Classic. His injury woes continued into the
following season whereas Gallagher broke his hand and missed a period of eight weeks as a result. The
2017–18 season was a career year for Gallagher, despite the Canadiens not qualifying for the
Stanley Cup playoffs. He was placed on a shutdown line along with centreman Tomáš Plekanec and led the team in even-strength points. Gallagher ended the regular season with a career-high 54 points and was nominated for the
King Clancy Memorial Trophy as a player who best exemplifies leadership qualities and community contributions. He was likewise named the recipient of the Canadiens' annual
Molson Cup honor as player of the year. For the
2018–19 season, Gallagher was once again named as team alternate captain with the inclusion of
Paul Byron. On February 21, 2019, Gallagher recorded his first career NHL hat-trick in a 5–1 win against the
Philadelphia Flyers. He played in his 500th career NHL game against the
Arizona Coyotes on October 30, 2019, registering a goal. On October 14, 2020, Gallagher signed a six-year, $39 million contract extension with the Canadiens. On May 31, 2021, Gallagher scored the critical first goal of Game 7 against the
Toronto Maple Leafs after the Leafs'
Mitch Marner accidentally passed the puck to him. The Canadiens ultimately won the game 3–1 to
complete their comeback from a 3–1 series deficit in the first round of the
2021 Stanley Cup playoffs. On June 1, Gallagher netted the game winner in Game 1 against the
Winnipeg Jets, whom the Canadiens
swept in the second round. On June 24, Gallagher had an assist on
Artturi Lehkonen's semifinal series winner in Game 6 overtime against the
Vegas Golden Knights, sending the Canadiens to their first
Stanley Cup Final in 28 years. The Canadiens lost the Final in five games to defending champions the
Tampa Bay Lightning. Following appearing in the
Stanley Cup Final, the
2021–22 season was a struggle for both Gallagher individually and the Canadiens overall. As a result of various injuries and a severe bout of
COVID-19, Gallagher collectively missed 26 games, and, after four consecutive years as one of the league's most dominant forwards during five-on-five play, would not score a goal at even strength until an April 5, 2022 game against the
Ottawa Senators. Newly appointed team head coach
Martin St-Louis expressed a desire to "rewire" Gallagher's approach to playing the game, encouraging him to "use up less physical energy by playing a bit more of a mental game." Finishing with just seven goals and 17 assists, Gallagher would later say that the main consolation of his worst season production wise to date was a four-month off-season that would enable him to recover his health and strength by working with his father Ian. While there was some discussion of Gallagher being named the team's new captain following the departure of
Shea Weber, this was ultimately given to
Nick Suzuki. Injury again caused Gallagher to miss significant playing time during the
2022–23 season whereas he would break his ankle blocking a shot in the team's twelfth game of the season and, despite attempting to play through, suffer yet another break in January 2023. As a result, Gallagher would be sidelined until the end of March. That same month, he scored his 200th career NHL goal in a game against the
Buffalo Sabres on March 27, tying the contest late in the third period. When discussing his perpetuation to injury at season's end, Gallagher said he would have to change his approach to his health, admitting "it's really hard to change your identity as a player and everything that I've always believed in. But, certainly, in the end I ended up missing more time due to it. That's something I'll have to think about and going forward probably be a little bit more honest with myself about." During the course of the
2024–25 season, Gallagher played in his 800th career NHL game on January 23, 2025 versus the
Detroit Red Wings. Days later, he surpassed
Jean-Guy Talbot on the Canadiens' all-time games played list to enter the top-20 across franchise history. The
2025–26 season saw Gallagher's role on the team decline in prominence, and on March 14 he was a healthy scratch from the lineup for the first time since January of 2013 during his rookie season. Days later, on March 17, he appeared in his 900th career NHL game, by that point 13th in games played in Canadiens franchise history. He ultimately finished the regular season with seven goals and 16 assists in 77 games. With the Canadiens qualifying to the
2026 Stanley Cup playoffs and facing the Tampa Bay Lightning in the first round, Gallagher was left out of the lineup for the first four games. He made his first appearance of the postseason in Game 5 on April 29, where he scored the opening goal on his first shift. The Canadiens would go on to a 3–2 win. ==International play==