Raynald Desjardins has admitted to his role in the death of reputed mob boss Salvatore Montagna in November 2011.
The killing is thought to be tied to a turf war for control of the Montreal Mafia.
Desjardins was initially charged with first-degree murder in connection with Montagna's death.
Four other men were also charged.
Desjardins was himself the suspected target of an assassination attempt in September 2011.
Two months later, Montagna, a former New York Mafia boss who came to Montreal and is thought to have tried to take over the city's Mob, was gunned down northeast of Montreal.
Montagna's body was found in the Assomption River.
Nicknamed "Sal The Iron Worker," Montagna owned and operated a successful steel business in the U.S.
The FBI once called him the acting boss of the Bonanno crime family – prompting one of New York's tabloids to dub him the "Bambino Boss" because of his rise to power in his mid-30s.