Mommy was considered a favourite for the festival's top Palme D'Or award, but it went to Winter Sleep by Nuri Bilge Ceylan.
The festival jury this year is led by Jane Campion and includes Willem Dafoe, Sofia Coppola, Gael García Bernal, Carole Bouquet, Leila Hatami, Jeon Do-yeon, Jia Zhangke and Nicolas Winding Refn.
Two other Canadian films were vying for the coveted Palme d'Or — David Cronenberg's Maps to the Stars and Atom Egoyan's The Captive.
Maps to the Stars was also recognized by the jury on Saturday with a best actress award for Julianne Moore.
Dolan’s Mommy follows a single mother looking after her 15-year-old son with ADHD.
The film is the 25-year-old’s fifth full-length feature.