Moray Walker, 50, shouted a racist insult to Mohammed Tanveer, 40, after the driver refused to pick him up after a night out at Edinburgh's Prison Officers Club.
The city's sheriff court heard how Walker - who supervises inmates at HMP Saughton - also struck out at his victim's motor and damaged Mr Tanveer's car door.
Walker was found guilty of committing a racially aggravated breach of the peace on March 18, 2013 at Edinburgh Sheriff Court yesterday.
Walker, also of Edinburgh, had entered a not guilty plea and claimed he would never use racist language towards a fellow human being.
But Sheriff Neil McKinnon said there was enough evidence available to prove Walker had broken the law.
Murray will be sentenced at Edinburgh Sheriff Court on November 8, 2013.