The 2009–10 Elitserien season turned out to be a spectacular breakthrough season for Larsson. He scored two goals in an Elitserien game against Timrå IK on 15 October 2009. By the end of the season, Larsson had tied the single-season record for the number of points in Elitserien among defencemen under age 18 with 17 points, a record that was set by Tomas Jonsson in 1977–78, who compiled 8 goals and 9 assists in 35 games. Larsson fought Timrå IK forward Daniel Corso in a game on 2 November 2010, receiving a cut eyebrow during the altercation. Larsson commented that he kept his gloves on because removing them during a fight in the Elitserien leads to an automatic one-game suspension. Although Larsson did not remove his gloves, both he and Corso were suspended for two games and received fines as a result of the fight. Larsson's play in the 2010–11 season earned him a nomination for the Elitserien Rookie of the Year award, which Mattias Ekholm would eventually win.
New Jersey Devils
Larsson was selected in the first round, fourth overall, by the New Jersey Devils in the 2011 NHL Entry Draft. Seen as a move towards rebuilding the Devils defence corps, Larsson was rated by the NHL Central Scouting Bureau as the top European-based prospect available in the draft. Larsson, who has been compared to the Detroit Red Wings' defenceman Nicklas Lidström, signed a three-year, entry-level contract with no bonuses at the NHL maximum of US$925,000 per year with the Devils on 15 July 2011, due to general manager Lou Lamoriello's disapproval of performance bonuses. He made the team in training camp and became the first 18-year-old defenceman to dress for the Devils in the regular season since Scott Niedermayer in the season, only the sixth under-20 defenceman to play for the Devils, and the first 18-year-old defenceman in the NHL since 2008, when Drew Doughty, Zach Bogosian and Luke Schenn all made their NHL debuts. Larsson's first NHL goal was scored on 11 November 2011 on a slapshot from the right slot against Tomáš Vokoun of the Washington Capitals. He was selected to the NHL All-Star Rookie SuperSkills Competition on 12 January 2012, along with teammate Adam Henrique; however, he chose to pull out due to a minor injury. He was replaced by Montreal Canadiens rookie defenceman Raphael Diaz. On 1 May 2012, Larsson scored his first Stanley Cup playoff goal against the Philadelphia Flyers in Philadelphia in his first career playoff game, becoming the first Devils rookie defenceman to score a goal in his first playoff game. Following the 2014–15 season, Larsson became a restricted free agent under the NHL Collective Bargaining Agreement. The Devils made him a qualifying offer to retain his NHL rights and on 5 July 2015, he filed for salary arbitration under the CBA. On 25 July, he signed a new six-year, $25 million contract with the Devils.
Edmonton Oilers
On 29 June 2016, Larsson was traded to the Edmonton Oilers in exchange for Taylor Hall. Larsson was named an alternate captain for the Oilers during the 2017–18 season.
Larsson's father, Robert Larsson, played 249 games with Skellefteå AIK from 1985 to 1995. Robert Larsson was drafted in the sixth round, 112th overall, by the Los Angeles Kings during the 1988 NHL Entry Draft, but never played in North America. In the 2008–09 season, Larsson played with his older brother Hampus in the J20 SuperElit, Sweden's junior league.
