[2][3] 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.

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.

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.