Halák was the 271st overall pick by the Canadiens in the 2003 NHL Entry Draft. He uses a mix of butterfly style and stand-up style.

 

Halák was called up to the main roster on February 15, 2007, after an injury to goaltender Cristobal Huet.

 

Halák played his first NHL game on February 18, 2007. He stopped 31 shots in a 3-2 win over the Blue Jackets in Columbus, Ohio. Prior to his call-up, Halák had the lowest goals against average in the American Hockey League (2.00) for the Hamilton Bulldogs. He is the fourth Slovakian born netminder to play in the NHL, joining Ján Lašák, Rastislav Staňa and Peter Budaj. On March 20, 2007, Halak made thirty saves to give the Montreal Canadiens a 1-0 victory over the Boston Bruins for his first NHL shutout. He recorded a second shutout, also against the Bruins on April 2nd, 2007. Halak's great play helped Montreal fight for a playoffs spot until the final day of the regular season. Halak played for Team Slovakia in the 2007 Men's World Ice Hockey Championships.

 

Halak had been assigned to the Hamilton Bulldogs for the 2007-2008 hockey season, but was re-called to the Montreal Canadiens in January 2008.