The Toronto Maple Leafs selected Sandin, a defenseman from Uppsala, Sweden, in the first round (No. 29) of the 2018 NHL Draft. It came after he after played five games with Rogle of the Swedish Hockey League in 2017-18 before coming to North America and scoring 12 goals in 51 games for Sault Ste. Marie of the Ontario Hockey League.
The Maple Leafs quickly signed Sandin and assigned him to Toronto of the American Hockey League for 2018-19. He scored six goals and finished with 28 points in 44 games.