Sandin has previously played for the Maple Leafs and Rögle BK of the Swedish Hockey League (SHL).

[2] He made his professional debut in playing five games for the Rögle BK of the Swedish Hockey League (SHL) before joining the Greyhounds for the 2017–18 season.

Sandin played his first NHL game against the Ottawa Senators on 2 October 2019, recording one assist in a 5–3 win.

[12] During the 2022–23 season, approaching the NHL trade deadline, Sandin was dealt by the Maple Leafs to the Washington Capitals in exchange for Erik Gustafsson and a 2023 first-round draft pick on 28 February 2023.

[15] Sandin has an older brother Linus who is a player for Rögle BK of the Swedish Hockey League (SHL).