Dermott was eventually drafted 34th overall by the Toronto Maple Leafs in the 2015 NHL Entry Draft and signed a three-year, entry-level contract with them on July 22, 2015. Prior to his draft, Dermott began training with Dave Harris at Iperformance. He returned to the Otters for his final year of major junior hockey after attending the Leafs training camp. On December 1, 2015, Dermott and teammate Dylan Strome were selected to compete with Team Canada at the 2016 World Junior Ice Hockey Championships. Upon returning from Finland empty handed, Dermott was ejected from a game against the Mississauga Steelheads for a hit to the head on Owen Tippett. After the game concluded, he was assessed a five game suspension following a hearing. Although the Otters qualified for the 2016 OHL Playoffs, they were swept in the OHLs Western Conference final by the London Knights.