Sandin was ranked the 11th best North American prospect according to the final NHL Central Scouting Report.[6] On 22 June 2018, Sandin was chosen in the first round, 29th overall, by the Toronto Maple Leafs in the 2018 NHL Entry Draft.[7] He later agreed to a three-year, entry-level contract with the Maple Leafs on 16 July.[8] Sandin began the 2018–19 season with the Toronto Marlies, the Toronto Maple Leafs' American Hockey League affiliate, after being cut from training camp.[9] During the season, Sandin set a new team record for longest point streak by a defenceman.[10] Sandin finished the 2018–19 season with six goals and 28 points in 44 regular season games. Sandin played his first NHL game against the Ottawa Senators on 2 October 2019, recording one assist in a 5–3 win.[11] He eventually recorded his first career NHL goal on 27 January 2020, in a 5–2 win over the Nashville Predators.[12]
On 6 March 2024, Sandin signed a five-year, $23 million contract extension with the Capitals,[14] carrying an AAV of $4.6 million through the 2028-29 season.[15]