Kapanen began the 2015–16 season with the Maple Leafs' American Hockey League (AHL) affiliate, the Toronto Marlies.
As the youngest player in the AHL, Kapanen played well in his role on a deep Marlies team that finished first in league standings.
He made his NHL debut on 29 February 2016 against the Tampa Bay Lightning as part of a night where four players would make their debut, including William Nylander.
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.
Despite finishing as the top seed, the Marlies were eliminated in the Eastern Conference Finals by the Hershey Bears.

Kapanen returned to the Marlies the following season and quickly became one of their best players.
Despite periods of injury, he scored at a pace of a point-a-game before an injury to Maple Leafs forward Nikita Soshnikov resulted in Kapanen being recalled to the Maple Leafs late in the season.
Kapanen took a role on the fourth line alongside Brian Boyle and Matt Martin, playing well in limited minutes.
He scored his first NHL goal on 8 April 2017 against the Penguins, the team that had drafted him.
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.
A week later, he scored with 8:07 left in double overtime in Game 2 of the Eastern Conference Quarterfinals as his Maple Leafs defeated the Washington Capitals 4–3 to tie the series at one game apiece.
The Leafs ended up losing the series in six games.

Kapanen was sent down to the Marlies to begin the 2017–18 season, but was recalled on 28 October 2017 after playing six games for the Marlies.
He was sent back down to the Marlies on 7 November, only to be recalled again on 12 December.
He had been selected for the 2018 AHL All-Star Game, but was replaced by Andreas Johnsson.
Kapanen recorded his first and only goal of the 2018 playoffs in Game 7 against the Boston Bruins, becoming the youngest player in NHL history to score a shorthanded goal in a Game 7.
The Maple Leafs lost the series 3–4.
Following the Maple Leafs' defeat, Kapanen joined the Finland senior national team at the 2018 IIHF World Championship.

After attending the Maple Leafs' training camp, Kapanen began the 2018–19 season NHL.
As teammate William Nylander remained unsigned and in contract negotiations with Toronto, Kapanen replaced his spot on the team's top line.
Kapanen finished the season with 20 goals and 44 points in 78 games.

On 28 June 2019, Kapanen signed a new three-year, $9.6 million contract with Toronto worth an annual average value of $3.2 million.
Kapanen began the 2015–16 season with the Maple Leafs' American Hockey League (AHL) affiliate, the Toronto Marlies. As the youngest player in the AHL, Kapanen played well in his role on a deep Marlies team that finished first in league standings. He made his NHL debut on 29 February 2016 against the Tampa Bay Lightning as part of a night where four players would make their debut, including William Nylander. 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. Despite finishing as the top seed, the Marlies were eliminated in the Eastern Conference Finals by the Hershey Bears.
Kapanen returned to the Marlies the following season and quickly became one of their best players. Despite periods of injury, he scored at a pace of a point-a-game before an injury to Maple Leafs forward Nikita Soshnikov resulted in Kapanen being recalled to the Maple Leafs late in the season. Kapanen took a role on the fourth line alongside Brian Boyle and Matt Martin, playing well in limited minutes. He scored his first NHL goal on 8 April 2017 against the Penguins, the team that had drafted him. 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. A week later, he scored with 8:07 left in double overtime in Game 2 of the Eastern Conference Quarterfinals as his Maple Leafs defeated the Washington Capitals 4–3 to tie the series at one game apiece. The Leafs ended up losing the series in six games.
Kapanen was sent down to the Marlies to begin the 2017–18 season, but was recalled on 28 October 2017 after playing six games for the Marlies. He was sent back down to the Marlies on 7 November, only to be recalled again on 12 December. He had been selected for the 2018 AHL All-Star Game, but was replaced by Andreas Johnsson. Kapanen recorded his first and only goal of the 2018 playoffs in Game 7 against the Boston Bruins, becoming the youngest player in NHL history to score a shorthanded goal in a Game 7. The Maple Leafs lost the series 3–4. Following the Maple Leafs' defeat, Kapanen joined the Finland senior national team at the 2018 IIHF World Championship.
After attending the Maple Leafs' training camp, Kapanen began the 2018–19 season NHL. As teammate William Nylander remained unsigned and in contract negotiations with Toronto, Kapanen replaced his spot on the team's top line. Kapanen finished the season with 20 goals and 44 points in 78 games.
On 28 June 2019, Kapanen signed a new three-year, $9.6 million contract with Toronto worth an annual average value of $3.2 million.
Kasperi Kapanen