Metropolitan Police officer Jonathan Marsh, 34, was called to a shop in Atlanta Boulevard, Romford, east London, on November 13 2022 by the shop worker, who said someone was damaging his shop sign.
Marsh and a colleague tried to find the suspect, but then mistakenly identified the shopkeeper as the culprit and bundled him to the ground, with Marsh then punching him in the back of the head.
The shopkeeper was handcuffed and put in the back of a police car, before the officers realised their mistake and de-arrested him.
He complained to the Met, and the force referred the case to watchdog the Independent Office for Police Conduct (IOPC) to investigate.
Marsh, from Canvey Island in Essex, was found guilty of common assault at City of London Magistrates’ Court on Monday, and will be sentenced on February 29.
IOPC regional director Charmaine Arbouin said: “Police officers may only use force when it is necessary, proportionate, and reasonable in the circumstances.
“There was no policing purpose or justification for the officer to strike the man, who posed no genuine risk.
“The shopkeeper was a law-abiding member of the public who had called the police to report a crime when he became a victim of mistaken identity.
“We appreciate cases of mistaken identity can happen, however Pc Marsh made no attempt to establish the facts and the situation escalated quickly when he immediately used unnecessary force on the man who posed no threat to him or his colleague.
“Now Pc Marsh has a conviction against his name.”
The Met Police said its internal misconduct process would now begin, and that Marsh remained on restricted duties.