He made his debut for Växjö Lakers in the Swedish Hockey League (SHL) in 2018, playing two games.

[4] Holmberg was demoted to the Toronto Marlies and played seven games with them before being recalled to the Maple Leafs.

[7] Holmberg took part in the Maple Leafs' 2023 training camp but failed to make the team.

[9] He made the Maple Leafs out of the 2024 training camp, playing on third line with Max Pacioretty and Nick Robertson.

[11][13] At the 2024 IIHF World Championship he won a bronze medal, while playing on a line with Lucas Raymond and André Burakovsky.