Muzzin played his early minor hockey in the Woodstock Minor Hockey Association before moving on to play AAA in Brantford, Ontario. He had previously played both soccer and hockey but at the age of 13 he committed to hockey. Muzzin was the first pick, 11th overall, by Sault Ste. Marie Greyhounds of the Ontario Hockey League in the 2005 OHL Priority Selection Draft from the Brantford 99'ers Minor Midgets. He did not play with the Greyhounds until late in 2006 after he required herniated disc surgery at age 16. Despite having played only 37 regular season games and 13 playoff games, he was drafted 141st overall in the 2007 NHL Entry Draft by the Pittsburgh Penguins. Although he was drafted, he went unsigned by the Penguins and re-entered the 2009 NHL Entry Draft but was not picked and returned for an overage year with the Greyhounds. In his final season in the OHL, Muzzin was named team captain and selected for the 2010 OHL All-Star Classic. Muzzin also won the Max Kaminsky Trophy for top defenceman in the league.
Professional
Los Angeles Kings
Muzzin signed as a free agent to a three-year, entry-level contract with the Los Angeles Kings on January 4, 2010. Following the Kings' training camp and pre-season games, Muzzin made the Kings' opening night roster for the 2010–11 season. However, on November 3, 2010, he was assigned to the Kings' American Hockey League affiliate, the Manchester Monarchs. Just six days later, on November 9, the Kings recalled Muzzin for a brief stint before again reassigning him to the Monarchs on November 23. shortly after the 2012 Stanley Cup Finals. In 2012, Muzzin was recalled during the 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. On February 24, 2020, Muzzin agreed to a four-year, $22.5 million contract extension with the Maple Leafs.
Muzzin became engaged to Courtney Fischer in December 2015, and they married on August 5, 2016, in London, Ontario. They have one child together, a daughter born in April 2019.
