Const. Jason Nevill was also found guilty of charges of fabricating evidence and obstruction of justice on Friday.
Nevill initially maintained that Jason Stern had attacked him with intent to resist arrest in November 2010.
However, charges against Stern, who is now 28, were dropped after video surveillance emerged appearing to show the officer attacking him without provocation and beating him even after handcuffing him.
The incident occurred after Stern's friend broke a Christmas ornament at the mall.
Stern was suing Nevill and the mall for $1 million, arguing he sustained permanent physical and emotional injuries from the beating.