2012 Stanley Cup playoffs and was on the roster as a healthy scratch when they won the Stanley Cup over the New Jersey Devils. He did not appear in any games that season. Although he did not qualify to have his name engraved on the Stanley Cup, he was nonetheless included in the team's celebratory picture. Muzzin scored his first NHL goal in his third game following the 2012–13 NHL lockout on January 26, 2013, against Jason LaBarbera of the Phoenix Coyotes. He went on to become a regular in the Kings' lineup and scored 7 goals and 9 assists in 45 regular season games, while adding 3 assists in 17 playoff games. He was also named NHL Rookie of the Month for March 2013. On July 12, 2013, Muzzin and the Kings agreed to a two-year contract extension. At the beginning of the 2013–14 season, Muzzin began as a third line pairing and was a healthy scratch for a series of games. However, he quickly continued to develop and, after being paired with defensive star Drew Doughty, he continued to earn more playing time on the top defensive pairing. Muzzin continued to earn minutes for the Kings and finished the regular season averaging 19:01 minutes per game, in addition to registering 5 goals and 19 assists in 76 regular season games. In the 2014 playoffs, he scored 6 goals and 6 assists in 26 games, averaging 23:23 minutes of ice time per game, including 37:18 in Game 5 of the 2014 Stanley Cup Finals against the New York Rangers. On October 15, 2014, Muzzin signed a five-year contract extension with the Kings. He went on to have a career-high 41 points in 76 games that season. On January 6, 2018, Muzzin was placed on injured reserve by the Kings. He was activated from injured reserve on January 18 after missing three games. During the 2018 playoffs, due to an upper-body injury, Muzzin played in only two games in a 4–0 series loss to the Vegas Golden Knights. Toronto Maple Leafs On January 28, 2019, with the Kings one point from last place in the league standings, Muzzin was traded to the Toronto Maple Leafs in exchange for forward Carl Grundström, the playing rights to Sean Durzi, and Toronto's first-round pick in the 2019 NHL Entry Draft. He finished the season with 37 points, split between the Kings and Leafs.