Darren Murphy smashed John Morrison, 50, in the face during an unprovoked attack in Glasgow city centre in December 2012.
Mr Morrison - who had been enjoying a works night out shortly before the assault - died almost instantly after striking his head on the ground.
Bank worker Murphy left his stricken victim in the street to go to a nightclub with friends.
Murphy told a trial last year that he acted in self defence - against man who was almost a foot smaller and double his age.
But, it emerged yesterday the 26-year-old made-up the claims in a bid to dupe the jury.
Murphy was back in the dock after earlier being convicted of culpable homicide.
Judge Norman Ritchie QC told him: "Fatefully John Morrison's journey home that night crossed the route taken by you and your friends.
"It is not entirely clear what caused the altercation, but it is pretty certain it was not much."
The judge said Murphy's decision to confront his victim was "without proper reason".
Murphy looked ashed-faced as he was lead handcuffed to the cells. Many of his friends and family broke down in tears.
Mr Morrison, the former LibDem leader of East Dunbartonshire Council, was attacked in the early hours of December 22 last year.
The lawyer had earlier been on a festive night out with colleagues from Glasgow City Council, where he worked as a legal manager.
His friend and work-mate Jennifer McMartin told the jury: "He was in really good form that night.
"He said he was in a good place in his life both professionally and personally having met his partner."
Mr Morrison died in the street despite medics coming to his aid. The ferocity of the blow had broken his eye socket and nose.
During the trial, Murphy, of the city's Bridgeton, denied he had punched Mr Morrison, who was just over five foot tall.
He instead claimed he had "hit him with a palm" to get him away.