Darren Murphy, 26, smashed John Morrison, 50, in the face during a motiveless attack in Glasgow city centre last December.
Former East Dunbartonshire leader Mr Morrison - who had been enjoying a works night out shortly before the assault - died almost instantly after striking his head on the ground.
Murphy, a bank worker, said he had been acting in self defence but his claims were rejected and he was found guilty of culpable homicide at the High Court in Glasgow.
First offender Murphy cried at the verdict while at least two female jurors also broke down. Murphy's bail was continued until sentencing in January.