Aittokallio was selected in the fourth round, 107th overall by the
Colorado Avalanche in the
2010 NHL entry draft. Despite ranked as the number one European Goalie before the draft, he was the seventh goaltender taken. In the
2009–10 season, Aittokallio made his debut in the
SM-liiga as a 17-year-old playing in one game for
Ilves. As well as representing Ilves in junior, Aittokallio spent a portion of the season on loan with
Lempäälän Kisa in the
Mestis league. On 12 April 2010, Aittokallio signed a two-year contract extension to remain with Ilves, his original team as a youth. In each season, he continued to split the year between Ilves youth team, LeKi and in the
2010–11 SM-liiga season, he played a career high 16 games for 5 wins. After fulfilling his junior eligibility and his commitment to Ilves, Aittokallio was signed to a three-year entry-level contract with the Avalanche on 11 May 2012. After 26 games with the Monsters, Aittokallio received his first NHL recall to the Avalanche during the
lockout shortened
2012–13 season, to fill the back-up position after an injury to starting goaltender
Semyon Varlamov on 9 April 2013. On 11 April 2013, he made his NHL debut against the
Los Angeles Kings, becoming the youngest since
Marc Denis to start for the Avalanche. Aittokallio failed to complete the game after suffering from dehydration with the game tied midway through the third period, and was replaced in the eventual shootout loss by veteran
Jean-Sébastien Giguère. In the
2014–15 season, Aittokallio suffered from injury and indifferent form, playing with the Monsters and the
Fort Wayne Komets of the
ECHL. Despite being tendered a qualifying offer by the Avalanche, Aittokallio opted to return to Finland to further his development in signing a two-year contract with Liiga club, Oulun Kärpät on 2 July 2015. Later in the off-season on 25 August 2015, it was announced that Aittokallio agreed to a one-year contract to keep within Avalanche organization, with a European clause allowing the continuance of his contract with Kärpät. After two seasons with Kärpät, Aittokallio left the club and Finland in signing a one-year deal with Czech club,
HC Sparta Praha of the
Czech Extraliga on May 13, 2017. In the 2017–18 season, Aittokallio had a successful stint the Czech Republic, collecting 15 wins in a starting role through 28 games. On April 13, 2018, Aittokallio returned to the Finnish Liiga, signing a one-year deal with his third topflight Finnish club,
Vaasan Sport. In the
2018–19 season, Aittokallio was unable to reproduce the heights of previous seasons statistically, collecting 9 wins in 30 games and finishing with 3.10 goals against average as Sport narrowly missed out on the wild-card round. Having left Sport, Aittokallio agreed to a two-year contract with his fourth Liiga outfit, Ässät Pori, on May 10, 2019. Following two seasons with Ässät, Aittokallio left the Liiga, in signing a one-year deal with newly promoted German club,
Bietigheim Steelers of the DEL, on 5 July 2021. Aittokallio remained with the Steelers for two seasons before returning to the Finnish Liiga, to commence the
2023–24 season, with
JYP Jyväskylä on a trial contract on 22 September 2023. He made 1 appearance with JYP before the conclusion of his trial contract and was later signed for the remainder of the season with Slovak Extraliga club, HK Nitra, on 13 November 2023. On 29 May 2025, Aittokallio signed a one-year contract to play with the Scottish-based
Glasgow Clan in the British
Elite Ice Hockey League. ==International play==