[1] During the 2009–10 season as a member of the Chicago Blackhawks, he became the first Finnish goaltender to win the Stanley Cup, and has also played for the San Jose Sharks, Dallas Stars, Pittsburgh Penguins, Florida Panthers, and Montreal Canadiens.

Dale Tallon, then the Blackhawks general manager, lauded him as one of the top European goaltending prospects and praised his size and athleticism.

[5] Niemi recorded his first NHL start and win several days later on 1 March, a 4–2 victory versus the Los Angeles Kings.

Niemi started the second game, held on 3 October 2009, where he stopped all 23 shots he faced against the Panthers to earn his first NHL shutout.

[6] Niemi compiled 26–7–4 record during the regular season and replaced Cristobal Huet as the team's starting goaltender.

Chicago walked away from the deal on 2 August due to salary cap restraints and signed veteran goaltender Marty Turco from the Dallas Stars for $1.3 million.

[17] The Stars signed Niemi, forming a partnership with fellow Finnish starting goaltender Kari Lehtonen, on a three-year deal worth $13.5 million on 29 June.

With the Stars missing the post-season and with the acquisition of goaltender Ben Bishop, Niemi was placed on waivers to be bought out from the remaining year of his contract on 26 June 2017.

[25] On 14 November, Niemi was claimed off waivers by the Montreal Canadiens, and became their official backup on 4 January 2018 after the team traded Al Montoya to the Edmonton Oilers.

This career resurgence earned him the Montreal chapter of the Professional Hockey Writers' Association's nomination for the Bill Masterton Memorial Trophy[28][29] and a one-year, $950,000 contract extension with the Canadiens.

[31] As an impending free agent from the Canadiens following the 2018–19 season, Niemi ended his 11-year NHL career, returning to Finland in signing a one-year contract with Jokerit of the Kontinental Hockey League (KHL) on 5 June 2019.