Following a break out year in the Liiga with Oulun Kärpät during 2021–22 season, Niemelä was signed to a future three-year, entry-level contract with draft club, the Toronto Maple Leafs, on 13 May 2022.[4] Niemelä returned to Oulun Kärpät for the 2022–23 season and appeared in 58 games, scoring 8 goals and 18 points. At the end of the season, he joined the Maple Leafs' American Hockey League (AHL) affiliate, the Toronto Marlies.[5] Niemelä attended the Maple Leafs' 2023 training camp, but was assigned to the Marlies to start the 2023–24 season.[6]
At the 2021 World Junior Ice Hockey Championships, Niemelä led all defensemen in points, accumulating two goals and six assists on the way to winning the bronze medal with Team Finland.[7][8] After the end of the tournament, he was named Best Defenceman by the IIHF Directorate.[9] He joined Team Finland again for the 2022 edition of the tournament. The team reached the gold medal game this time, playing against Team Canada, and Niemelä played a pivotal role in the game's most famous moment, when his potentially game-winning shot into a wide open Canadian net was knocked out of midair by Canadian captain Mason McTavish, preventing Finnish victory. The Finns won the silver medal.[10]