Dubas, who has served as assistant GM since 2014, will replace Lou Lamoriello, whose contract expired at the end of this season and who will transition into a senior advisor role.
The 32-year-old Dubas, an advocate of analytics, has been considered a rising star in the NHL and was approached by several teams to interview last year. The Colorado Avalanche asked for permission to speak with Dubas about a position last spring, but the Maple Leafs blocked the move.
The Maple Leafs set franchise records for points (105) and wins (49) in the regular season, having made back-to-back playoff appearances for the first time since a six-season run from 1999 to 2004.
As assistant GM, Dubas was responsible for player personnel, managing the team's farm system as well as the team's research and development department. He also served as GM of the Toronto Marlies, the team's AHL affiliate that had the most regular-season points for the second time in three seasons.
Before joining the Leafs, Dubas was GM of the Sault Ste. Marie Greyhounds in the Ontario Hockey League. He took that position at age 25, making him the second-youngest GM in OHL history. Dubas was a player agent between 2006 and 2011; he was the youngest agent ever certified by the NHLPA.
Dubas got the nod over the Leafs' other internal candidate: 57-year-old Mark Hunter, who is also an assistant general manager.
The Maple Leafs have gone young before in the front office. In 1988, Gord Stellick became the league's then-youngest general manager at 30.
John Chayka became the youngest general manager in NHL history in May 2016 when, at 26, he was promoted by the Arizona Coyotes.
ESPN's Emily Kaplan and The Associated Press contributed to this report.