During the 2012–13 season, he also spent 13 games with the senior KalPa team in the SM-Liiga, Finland's top professional league, playing alongside his father.

[8] Entering his draft year as one of the top European prospects,[9] Kapanen spent most of the season with KalPa, scoring 7 goals in 47 games for a team that finished last in the league.

[10] He was expected to be an impact player for the Finland under-20 team at the 2014 World Junior Championship but was injured in the last practice before the tournament began and was unable to participate.

[11] The lost opportunity did not affect Kapanen's draft stock as he finished atop both the mid-season and final Central Scouting rankings amongst European-based skaters.

[16] On 1 July 2015, Kapanen was traded to the Toronto Maple Leafs in a blockbuster, multi-player deal involving star forward Phil Kessel.

[17] At the 2016 World Junior Ice Hockey Championship, Kapanen scored the game-winning goal in overtime against Russia to win the gold medal.

[18] Kapanen would play nine games for the struggling Maple Leafs, recording no points before being returned to the Marlies for the team's 2016 Calder Cup playoff run.

[20] The goal was a vital one in a game that saw the team clinch a spot in the Stanley Cup playoffs for the first time since 2013, a remarkable feat considering the Maple Leafs finished last in the entire league the previous season.

As teammate William Nylander remained unsigned and in contract negotiations with Toronto, Kapanen replaced his spot on the team's top line.

[38] Kapanen lived in Farmington, Connecticut, and Cary, North Carolina, prior to his father Sami's trade from the Hartford Whalers/Carolina Hurricanes franchise.

In September 2023, Finnish public broadcaster Yle reported Kapanen was suspected of aggravated drunk driving, with the case to be heard in district court in February 2024.