Darren Murphy hit John Morrison, 50, in the face during an unprovoked attack in Glasgow city centre in December 2012.
Mr Morrison, the former Liberal Democrat leader of East Dunbartonshire Council, who had been enjoying a works night out shortly before the assault, died almost immediately after striking his head on the ground. The ferocity of the blow had also broken his eye socket and nose.
Murphy, who is 6ft 1in, left his 5ft victim lying in the street to go to a nightclub with friends.
The 26-year-old, of Bridgeton, Glasgow, had told a trial last year that he had been acting in self defence against a man who was almost a foot smaller and double his age. But it emerged yesterday Murphy made up the claims in an attempt to trick the jury.
During the trial, Murphy had denied he had punched Mr Morrison, instead claiming he had "hit him with a palm" to get him away.
Murphy insisted he had "felt threatened" by Mr Morrison that night. However, Murphy told social workers compiling a pre-sentencing report he had "fabricated" his evidence.
The killer said he had not felt "scared" of Mr Morrison and that he had made it up as he was worried about being convicted and sent to jail.
Murphy was back in the dock for sentencing 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 the decision to confront his victim was "without proper reason".
Murphy looked ashed-faced as he was led handcuffed to the cells. Many of his friends and family broke down in tears.
Mr Morrison, of Milngavie, was attacked in the early hours of December 22. The lawyer had earlier been out with colleagues from Glasgow City Council, where he worked as a legal manager.
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 professionally and personally having met his partner David."
The court was shown CCTV footage of Mr Morrison later in Argyle Street, close to the St Enoch Centre.
Murphy and his friends were on the same street heading to the Arches nightclub.
Murphy's friend Samantha Mulgrew claimed during the trial that Mr Morrison suddenly appeared next to her mumbling, which gave her a "fright".
For some reason, Murphy and his group then turned on the solicitor.
CCTV showed the mob, who had been drinking, heading towards him before one pushed Mr Morrison as he appeared to be walking away.
Seconds later - and out of shot - Murphy pole-axed Mr Morrison by punching him in the face.
But, instead of checking how the father-of-one was, Murphy carried on to the nightclub. Mr Morrison died in the street, despite medics coming to his aid.
A source close to the case said: "Murphy's lucky he will meet his relatives again - those close to John Morrison are not so fortunate."