Halák earned his first career NHL shutout on 20 March 2007, making thirty saves to keep the Canadiens' rival Boston Bruins off the board with a 1–0 victory.

Halák went on to be named the Canadiens' Molson Cup Player of the Year for the season and was designated the starter heading into their first-round playoff match-up against the Presidents' Trophy-winning Washington Capitals.

[2] The win was attributed largely to Halák's play as he turned aside 131 of Washington's 134 shots in games 5 through 7, frustrating the league's top scoring team from the regular season.

[3] After defeating the Pittsburgh Penguins in seven games, the Canadiens fell to the Philadelphia Flyers in the Eastern Conference Final with a 4–1 series loss.

After weeks of media speculation the Canadiens chose Price and on 17 June, controversially traded Halák to the St. Louis Blues in exchange for two prospects, center Lars Eller and right-winger Ian Schultz.

Halák would earn his first shutout with the team on 23 October 2010 at home against the Penguins, stopping 31 shots in a 1–0 overtime victory.

[9] Halák did not appear in a game for the Sabres and was ultimately dealt along with a third round draft pick to the Washington Capitals in exchange for Michal Neuvirth and Rostislav Klesla at the trade deadline.

[11] As pending unrestricted free agent, Halák's rights were traded by the Capitals to the New York Islanders on 1 May 2014 for a fourth round pick in the 2014 draft.

Halák made his return to Long Island on March 19 and got a tribute video played for him along with a standing ovation from the fans in attendance.

[25] On 15 August 2020, Halák was named starter by the Bruins for the rest of the 2020 Stanley Cup playoffs after Rask left the team for personal reasons before the third game of the first-round series against the Carolina Hurricanes.

[26] Following his third season with the Bruins, Halák left as a free agent and was signed to a one-year, $1.5 million contract with the Vancouver Canucks on 28 July 2021.

Halák's first senior international experience came in 2007 as he played in the 2007 Men's World Ice Hockey Championships, starting 2 games and earning a 1–1 record.

[34] In February 2010, Halák was named the starting goaltender for the Slovak national team in the 2010 Winter Olympics, along with future Bruins defensive teammate Zdeno Chára.