He began his North American professional career with Chicago's AHL affiliate, the Rockford IceHogs, where he would spend the majority of the 2008-09 season. He made his NHL debut on February 27, playing one period in a 5-4 loss to the Pittsburgh Penguins. Niemi made his first NHL start several days later on March 1, a 4-2 victory versus the Los Angeles Kings.

In 2009-10, Niemi made the Blackhawks out of training camp. He started the second game of season in Helsinki, stopping all 23 shots he faced against the Florida Panthers, earning his first NHL shutout in his native Finland.

As the season wore on, Niemi outplayed counterpart Cristobal Huet and going into the playoffs he was regarded as Chicago's number one goaltender.

The season would end triumphantly for Niemi and the Blackhawks when they defeated the Philadelphia Flyers in six games to capture the club's first Stanley Cup championship since 1961, with Niemi providing solid play in the Hawks net.

In the summer of 2010, it became clear that Niemi would not be returning to Chicago to help defend the championship. He signed a one-year deal with the San Jose Sharks on September 2.

After playing in four seasons with the San Jose Sharks, Niemi's rights were traded to the Dallas Stars for a seventh round draft pick on June 17, 2015. Two days later, Niemi signed a three year contract with the Stars.

In two seasons with the Stars, Niemi posted goals-against-averages of 2.67 and 3.30. Following the 2016-17 season, the club decided to buy out the final year of his contract making the goaltender a free agent on July 1, 2017. He subsequently signed a deal with the Pittsburgh Penguins. Niemi would struggle to find his game in Pittsburgh and he was waived by the Penguins after just three games. He was then claimed by the Florida Panthers, but was waived once more after just two appearances. He was then picked up by the Montreal Canadiens.

On the international stage Niemi was part of the Finnish national team that won a bronze medal at the 2014 Winter Olympics.