Borgman signed a two-year, entry-level contract with the Toronto Maple Leafs on May 16, 2017, after he had 15 points (five goals, 10 assists) in 45 games with HV71 in the Swedish Hockey League, winning the Rookie of the Year award. He also appeared in 14 playoff games and tied for the lead among defenseman with 10 points (two goals, eight assists) to help HV71 win the SHL championship.
Borgman made his NHL debut in the Maple Leafs' season-opener on Oct. 4, 2017, getting an assist in a 7-2 win against the Winnipeg Jets. He scored his first NHL goal in a 3-2 loss to the San Jose Sharks on Oct. 30. In 48 games, Borgman had 11 points (three goals, eight assists) with the Maple Leafs before he was loaned to Toronto of the American Hockey League on Feb. 5, 2018.