Slafkovsky was selected by the Canadiens with the No. 1 pick in the 2022 NHL Draft. The first player born in Slovakia to be selected with the first pick of an NHL draft scored his first NHL goal in a 6-2 win against the Arizona Coyotes at Bell Centre on Oct. 20, a wrist shot to give the Canadiens a 4-0 lead at 8:17 of the second period.

He was given a standing ovation when the goal was announced, which was also his first NHL point in his fifth game.

"I don't even remember how I scored the goal," Slafkovsky said. "I just went to celebrate because I think that's the best part of the goal, the \[celebration\]. I was focused more on the celebration than how I scored.

"It's probably the best crowd, and I think there's nothing better that could happen after \[my\] first NHL goal than what happened out there. It's amazing."

Slafkovsky enjoyed a breakout second season in the NHL. He set a new Canadiens record for most points scored by a teenager in a single season, surpassing Mario Tremblay's 39 (21 goals, 18 assists) in 63 games in 1974-75, getting No. 40 with an assist on Suzuki's goal in a 2-1 win at the Colorado Avalanche on March 26, four days before his 20th birthday. He was the first teen with point streaks of eight and nine games and had a team-high 19 points (seven goals, 12 points) in the final 19 games of the regular season.

That breakthrough season earned Slafkovsky an eight-year, $60.8 million contract he signed with the Canadiens on July 1, 2024. His first goal of the 2024-25 season, during Montreal's 6-3 loss to the Pittsburgh Penguins at Bell Centre on Oct. 14, was his 64th NHL point, tying him with Guy Lafleur for sixth-most by a Canadiens player before the age of 21.

Slafkovsky scored the first goal of the Olympic Winter Games Milano Cortina 2026 and had three points (two goals, one assist) for Team Slovakia in a 4-1 win against Team Finland on Feb. 11. His eight points (four goals, four assists) were the third most by a Canadiens skater in an Olympic tournament with NHL players behind Saku Koivu (11 at the 2006 Torino Games and 10 at Nagano in 1998). He scored a power-play goal in a 3-2 win at the Ottawa Senators on March 11, 2026, for his 164th point, passing Henri Richard (163) for the most points in Canadiens history by a player before age 22.

Slafkovsky became the first Slovakian player with 30 goals in a season since Marian Hossa had 30 in 2013-14 when he scored No. 30 with 1:04 remaining in the third period to lift the Canadiens to a 2-1 win against the Tampa Bay Lightning at Bell Centre on April 9, 2026.

After playing for teams near his hometown of Kosice, Slafkovsky moved to Finland when he was 14 to play for TPS, and in 2019-20 played for TPS' teams in Under-16, Under-18 and Under-20 leagues. In 2021-22, he made his debut in Liiga, the top professional league in Finland, where he had 10 points (five goals, five assists) in 31 games, and seven points (two goals, five assists) in 18 playoff games,

Slafkovsky helped Slovakia to a second-place finish at the 2021 Hlinka Gretzky Cup with nine points (three goals, six assists) in five games. He was named the most valuable player at the 2022 Beijing Olympics after scoring a tournament-best seven goals and helping Slovakia win the bronze medal, the country's first Olympic medal in men's hockey.

Slafkovsky finished his season leading Slovakia at the 2022 IIHF World Championship with nine points (three goals, six assists) in eight games.