Bruce Mirabella, 51, was checking out at a Bartlett Jewel-Osco last July when he punched the worker in the face and then kicked him, police said. The grocery store is located on Sterns Road.
Police took him into custody a short time after the attack, and he was charged.
He was found guilty of aggravated battery in the incident.
"If these counts charged him with being a jerk, there's no doubt he's guilty," a DuPage County judge said, according to ABC 7 Chicago's news partner the Daily Herald.
Mirabella is next due in court April 6 for sentencing. His bond was revoked after his conviction, and he was taken into custody.