It happened on Cadieux Road in l'Ange-Gardien, Que., about 30 kilometres northwest of Ottawa, at about 6 p.m. ET.
The boy had been in the arms of his father moments before, according to MRC des Collines police. He wanted to play on the ground, so his father let him go.
The boy then quickly approached the riding mower without anyone seeing him, police said, and his foot was severed.
The boy was taken to hospital for treatment, police said.
His uncle was also taken to hospital for shock.