Toronto fired former general manager Dave Nonis in April. Maple Leafs assistant GM Kyle Dubas and director of player personnel Mark Hunter shared general manager duties during the search for Nonis' replacement.
Lamoriello spent 28 years in the Devils organization, serving as GM until moving into the president role exclusively when the team hired Ray Shero in May.
He also stepped behind the bench for three stints as the Devils' interim head coach.
“Lou Lamoriello created and defined what it meant to be a New Jersey Devil,” co-owner Josh Harris said in a statement.
"His brilliance in shaping this franchise into one of the most storied and celebrated organizations in sport will make him a New Jersey Devil for life. He represented this organization, our current and former players, the state of New Jersey, and the greatest fans in the National Hockey League in a manner that exemplified character, class, and dignity."
The Maple Leafs will provide the Devils with a third-round draft pick as compensation for Lamoriello's hiring, Tom Gulitti of The Record reports.
The Devils made the playoffs 21 times during Lamoriello's tenure, winning the Stanley Cup three times.
Maple Leafs president Brendan Shanahan was the first player selected by Lamoriello after becoming Devils GM in 1987. New Jersey drafted the Hall of Famer second overall.