[1] Leinonen was selected 41st overall by the Buffalo Sabres in the 2nd round of the 2022 NHL Entry Draft.

[2][3] After playing for Ilves in his youth, Leinonen moved to JYP's Under-16 program.

[6] Going into the 2022 NHL Entry Draft, Leinonen was regarded as the top goaltending prospect[6] and was ranked as the top European goalie by NHL Central Scouting.

[8][9] Leinonen represented Finland at the 2020 Winter Youth Olympics,[10] where he played 10 games with a .877 save percentage.

[11] Leinonen represented Finland at the 2022 IIHF World U18 Championships,[12] where he recorded a 2.61 goals against average and a .897 save percentage in 5 games played.