De Vito was found unconscious in his cell at the federal prison in Donnacona in July.
The 46-year-old, who was associated with the Rizzuto clan, was arrested in 2010 and convicted of conspiracy to import drugs and gangsterism.
According to the coroner's report, tests found toxic-levels of cyanide in De Vito's blood and stomach.
De Vito was also the husband of Adele Sorella, who is in prison for killing their two daughters while he was on the run.
On June 24, she was convicted of first-degree murder in the March 2009 deaths of the children.