The smooth-skating defenseman for the Florida Panthers had his second straight strong season in 2017-18, finishing with an NHL career-high 27 points (10 goals, 17 assists) and a team-high 137 blocked shots in 81 games. The Pointe-Claire, Quebec, native ended a season-opening, 28-game goal drought Dec. 11, 2017, when he scored in overtime of a 2-1 victory against the Detroit Red Wings, sending him on his way to his breakout NHL season.

Matheson, the Panthers' first-round pick (No. 23) in the 2012 NHL Draft, earned Florida's nomination for the 2017-18 King Clancy Memorial Award, given to the player "who best exemplifies leadership qualities on and off the ice and has made a noteworthy humanitarian contribution in his community." Matheson ran clinics at local schools and volunteered at food trucks in the aftermath of Hurricane Irma in September among other community activities.

On Oct. 9, 2017, two games into his second full NHL season, he signed an eight-year contract with the Panthers.

Matheson scored 17 points (seven goals, 10 assists) in 81 games for Florida during 2016-17, his rookie season, and had the most even-strength ice time among Panthers defensemen (1,453:51). He was second in the NHL among rookie defensemen in goals and in shots (179) behind Zach Werenski of the Columbus Blue Jackets.

Matheson has also played for the Pittsburgh Penguins and Montreal Canadiens. He signed a five-year, $30 million contract (average annual value of $6 million) with the Canadiens on Nov. 28, 2025.

He played most of 2015-16, his first full pro season, with Portland of the American Hockey League. He made his NHL debut against the Winnipeg Jets on Feb. 20, 2016. He was scoreless in three regular-season games, then had one assist in five Stanley Cup Playoff games against the New York Islanders in the Eastern Conference First Round.

At the 2016 IIHF World Championship, Matheson scored six points (two goals, four assists) in 10 games, helping Canada finish first. He was named the tournament's best defenseman. Matheson also helped Canada finish second at the 2017 Worlds with seven points (one goals, six assists) in 10 games.

Before signing with the Panthers, Matheson played three seasons at Boston College, where he was captain in 2014-15.

Matheson met his wife Emily Pfalzer, a defenseman who helped the United States win gold at the 2018 Pyeongchang Olympics, while at Boston College. The two married July 20, 2019.