Const. Jason Nevill was convicted of assault causing bodily harm, along with obstruction of justice and fabricating evidence against his victim.
Nevill initially maintained that Barrie resident Jason Stern had attacked him with intent to resist arrest in November 2010.
However, charges against Stern, now 28, were dropped after video surveillance emerged showing the officer attacking him without provocation and beating him even after handcuffing him.
In convicting the officer, the judge said he rejected Nevill's claims that he thought Stern had a knife and had kicked him in the groin based on the video.
At the same time, he said Stern was a forthright witness.
"The force used by Const. Nevill right from the outset was excessive,'' the judge said.
His version was exaggerated and did not match the surveillance video, which showed the officer's account to be a fabrication, he 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.
Nevill, a 14-year veteran of the force, has been suspended from active duty pending the outcome of the trial.
© 2013 The Canadian Press