Michael Theriault, who was off-duty at the time of the attack, was charged with his brother Christian of aggravated assault and obstruction of justice after beating Dafonte Miller and hitting him with a metal pipe.
Miller, then 19, lost the use of an eye in the attack on December 28, 2016.
A judge from the Ontario Superior Court found Michael Theriault guilty of a lesser charge of common assault, while his brother was acquitted.
Both men had pleaded not guilty and said they acted in self-defense, claiming Miller was robbing cars prior to the altercation.
The ruling comes against a backdrop of demonstrations denouncing racism and police violence as part of widespread protests over the killing of African-American George Floyd in Minneapolis.
Theriault is due to be sentenced on July 15.
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