Junior 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 the
Sault Ste. Marie Greyhounds of the
Ontario Hockey League (OHL) 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. 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 (AHL) 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. In 2012, Muzzin was recalled during the
2012 Stanley Cup playoffs as a
Black Ace, 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 Final 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. During the
2021–22 season, Muzzin suffered two concussions within a span of a month. The first concussion occurred on January 15, 2022, in a game against the St. Louis Blues, when Muzzin was checked into the boards by Blues forward
Klim Kostin. He missed seven games as a result. The second happened after a collision with
Chris Wideman of the Montreal Canadiens on February 21, 2022 (his 33rd birthday). Muzzin did not return to the ice until April 2022 and played for the Leafs in the playoffs. The
following season, after appearing in only four games, Muzzin injured his spine after colliding with Arizona Coyotes forward
Clayton Keller, and was placed on the injured reserve once again. He was later ruled out for the rest of the season on February 22, 2023. Muzzin remained on injured reserve to start the
2023–24 season, with new Maple Leafs general manager
Brad Treliving confirming in late September 2023 that Muzzin would miss the entire season due to his spinal injury suffered the prior year. Despite not officially retiring, Muzzin later joined the Maple Leafs' front office in early October as a professional scout. He was elevated the following year to their player development division. ==International play==