Amateur Gagner played
midget hockey with the
Toronto Marlboros of the
Greater Toronto Hockey League (GTHL) in 2004–05. He scored 173 points and was awarded the Buck Houle Award, a Marlboros team award given for outstanding on-ice performance and leadership. The same season, he made his
junior debut, playing 13 games for the
Milton Icehawks of the
OPJHL, scoring 15 points. His dad was the coach of his minor hockey career. In 2005–06, he joined the
Sioux City Musketeers of the
United States Hockey League (USHL) and scored 46 points in 56 games, second in team scoring. Along the way, Gagner played with future NHLer
John Tavares on the Marlboros and later the Icehawks. As early as 2002, the pair also honed their skills and became friends on a backyard rink built by Sam's dad Dave in Oakville. Gagner originally committed to play hockey at the
collegiate level for the
University of Wisconsin–Madison, but later decided to stay closer to home by playing for a Canadian
major junior hockey team. Considered a first-round talent, the
Ontario Hockey League (OHL)'s
London Knights selected him in the fourth round of the 2005 OHL entry draft on a flyer, as his commitment to play college hockey deterred many OHL teams from drafting him. Joined by future NHL stars
Patrick Kane and
Sergei Kostitsyn on the Knights' top line in
2006–07, Gagner scored 118 points in 53 games, fifth in OHL scoring. He also captained Team Burns/Bergeron (Red) in the 2007
CHL Top Prospects Game to a 5–3 victory over Team Bowman/Demers (White).
Professional Edmonton Oilers Gagner was drafted in the first round, sixth overall by the
Edmonton Oilers in the
2007 NHL entry draft. On 1 October 2007, he was then signed to a three-year, entry-level contract with the Oilers. He played his first NHL game on 4 October in a 3–2 shootout victory over the
San Jose Sharks and earned his first NHL point, assisting on a goal scored by defenceman
Tom Gilbert. On 20 October, Gagner scored his first career NHL goal against
Miikka Kiprusoff of the
Calgary Flames. As the NHL's youngest player in
2007–08, he tallied 49 points in 79 games playing between wingers and fellow rookies
Andrew Cogliano and
Robert Nilsson on a combination dubbed the "Kid Line." During the season, Gagner participated in the 2008
NHL YoungStars Game in
Atlanta as part of
All-Star weekend and was also named the NHL Rookie of the Month for February. As part of a month-long scoring stretch in which he scored 13 points in 12 games, he also established an Oilers team record for the longest assists streak by a rookie with nine in eight straight games. The next season,
2008–09, Gagner recorded his first career NHL
hat-trick and added an assist for a four-point game in an 8–1 win against the
Colorado Avalanche on 19 March 2009. He finished his sophomore season with 16 goals and 41 points in 76 games. In the following two campaigns, he posted 41- and 42-point efforts. On 9 March 2011, Gagner severed a tendon in his left hand while facing the
Washington Capitals when teammate
Ryan Jones caught him with a skate blade while jumping over the boards. Requiring surgery, Gagner was sidelined for the remainder of the
2010–11 season, as well as one month into the following campaign, making his return to the Oilers line-up on 22 October 2011. Several months later, on 2 February 2012, Gagner scored four goals and four assists against the
Chicago Blackhawks, tying the team record of eight points in a game, previously set by
Wayne Gretzky and
Paul Coffey. The feat also made him the eighth NHL player of all time to score
eight or more points in a single game, and the first since
Mario Lemieux did it in 1989. Two days later, Gagner scored two goals and added an assist in the first period against the
Detroit Red Wings, setting a new Oilers record with 11 consecutive points, previously held by Gretzky, who scored ten consecutive points twice his career. Gagner finished the season with 47 points over 75 games, with nearly a quarter of his output having occurred in that two-game stretch. During the
2012–13 NHL lock-out, Gagner signed with
Klagenfurter AC of the
Eishockey Liga in
Austria. On 22 July 2013, shortly after the end of the
2012–13 season, the Oilers signed Gagner to a three-year contract extension worth an average annual value of $4.8 million.
Trade to Arizona On 29 June 2014, the
Tampa Bay Lightning acquired Gagner in exchange for
Teddy Purcell. Gagner, however, only spent an hour as a member of the Lightning before then being acquired by the
Arizona Coyotes, along with
B. J. Crombeen, in exchange for a sixth-round pick in the
2015 NHL entry draft. During the Coyotes' training camp ahead of the
2014–15 season, Arizona head coach
Dave Tippett experimented utilizing Gagner on the right wing instead of his natural centre position, where he played in a pre-season, 5–4 shootout victory over the
Los Angeles Kings. With a fresh start, former with Arizona, Gagner found chemistry playing on a line with centre
Martin Hanzal; the two combined for 16 points in just five games in December and January 2015.
Philadelphia Flyers/Columbus Blue Jackets On 27 June 2015, Gagner was traded to the
Philadelphia Flyers along with a conditional third (2017) or fourth (2016) round draft pick in return for
Nicklas Grossmann and the contract of
Chris Pronger. Coyotes general manager
Don Maloney explained the trade as being due to a belief in the organization that Gagner "couldn't play centre at the NHL level". On 1 August 2016, Gagner signed a one-year deal with the
Columbus Blue Jackets. Following a resurgent season with the Blue Jackets where he set a career-high in points with 50, Gagner signed a 3-year $9.45 million contract with the
Vancouver Canucks.
Vancouver Canucks and return to Edmonton Prior to the
2018–19 season, after attending the Canucks training camp and pre-season Gagner was placed on waivers by the Canucks on 1 October 2018. He cleared waivers the next day, and was loaned in order to be closer to his family to the
Toronto Marlies, affiliate of the
Toronto Maple Leafs. Gagner scored 15 points in 15 games with the Marlies before he was recalled by the injury-struck Canucks on 18 November 2018. He registered 1 goal and 3 points in 7 games before Vancouver returned to health and he was returned to the Marlies on 4 December 2018. Through 43 games with the Marlies, Gagner was among the team's top scorers with 37 points. He was traded by the Canucks in return to his original draft club, the Edmonton Oilers, in exchange for
Ryan Spooner on 16 February 2019. During the
2019–20 season, Gagner recorded five goals and seven assists in 36 games for the Oilers.
Detroit Red Wings On 24 February 2020, Gagner was traded at the NHL trade deadline from the Oilers, along with second-round draft picks in
2020 and
2021, to the
Detroit Red Wings in exchange for
Andreas Athanasiou and
Ryan Kuffner. He added one goal in 6 games with the Red Wings before their season was prematurely ended due to the
COVID-19 pandemic suspending play. On 26 September 2020, the Red Wings signed Gagner to a one-year, $850,000 contract extension. On 28 July 2021, the Red Wings signed Gagner to a one-year contract extension.
Winnipeg Jets As a free agent from the Red Wings, Gagner continued his career by signing for his 16th season in the NHL in agreeing to a one-year, league minimum $750,000 contract with the Winnipeg Jets on 2 September 2022. In a depth forward role for the Jets in the
2022–23 season, Gagner played his
1,000th NHL career game on 29 December 2022, against the
Vancouver Canucks. He contributed with 8 goals and 6 assists for 14 points in 48 regular season games before he was ruled out for the remainder of the campaign with season ending surgery to both hips on 16 March 2023.
Third tenure with Edmonton As a free agent following his lone season with the Jets, Gagner opted to extend his career by accepting an invitation to return to the Oilers on a
professional tryout contract in preparation for the
2023–24 NHL season on 29 August 2023. Continuing his rehab from his double hip surgery, Gagner did not appear in any pre-season games with the Oilers, and was later signed to a one-year AHL contract with the Oilers' affiliate, the
Bakersfield Condors, for the
2023–24 AHL season on 23 October 2023. However, just eight days after signing with Bakersfield, Gagner was signed to a one-year, two-way NHL contract by the Oilers on 31 October, marking his third tenure with the team. In his first game with Edmonton since 2020, Gagner scored two goals, but the Oilers lost to the
Dallas Stars 4–3.
Belleville Senators and retirement As an unsigned free agent leading into the mid-point of the
2024–25 season, Gagner opted to continue his professional career by signing a
professional tryout (PTO) contract with AHL outfit, the
Belleville Senators, affiliate to the
Ottawa Senators, on 24 January 2025. Several months later, on 15 May, Gagner joined the Ottawa Senators' front office as director of player development, ending his playing career. ==International play==