Const. Jason Nevill was convicted of assault causing bodily harm, obstruction of justice and fabricating evidence against Jason Stern.
"This was a brutal assault," Stern said.
"Nevill assaulted me, with no provocation."
The officer initially maintained that Stern, now 28, was intoxicated and had attacked him with intent to resist arrest at a mall in November 2010.
However, charges against Stern were dropped after video surveillance emerged showing the officer attacking him without provocation, and beating him even after handcuffing him, leaving a pool of blood on the floor.
In convicting the officer, Ontario court Judge Lorne Chester rejected Nevill's claims that he thought Stern had a knife and had kicked him in the groin.
The video showed otherwise, Chester said in rendering his verdict, which took more than an hour to read. At the same time, he called Stern a forthright and reliable witness.
"The force used by Const. Nevill right from the outset was excessive," Chester said.
Nevill's version was exaggerated and did not match the surveillance video, which showed the officer's account to be a fabrication, the judge said.
Stern, he said, was "rag-dolled" by the burly officer.
The incident occurred at the mall after Stern's friend broke a Christmas ornament.
Stern is suing Nevill and the mall for $1 million, arguing he sustained permanent physical and emotional injuries from the beating.
When he took the injured Stern to hospital, Nevill poked fun at the victim for the amount of blood that was on the back of his cruiser, according to Stern's lawyer, Bernard Keating.
"Our client was subjected to the worst possible treatment by a violent, lying public servant," Keating said.
"With this case behind us, Jason can now focus on moving on with his life and obtaining appropriate damages for his continuing suffering."
Nevill, a 14-year veteran of the force, was suspended from active duty pending the outcome of the trial and remains on bail.
Barrie Police Chief Kimberley Greenwood told a late afternoon news conference she has spoken to Stern.
"I have extended, on behalf of all of the members of the Barrie Police Service, our apologies," Greenwood said.
She described Stern's beating as "a rare incident," adding "we have many, many officers on this service that are representative of our community and representative of our values."
Sentencing will occur Oct. 17 after which Nevill's lawyer said they would make a decision on whether to appeal.