Darren Murphy hit John Morrison in the face during a motiveless attack in Glasgow city centre last December.
Mr Morrison, 50, the former Lib-Dem leader of East Dunbartonshire Council, 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.
He told a jury that he had been acting in self defence, against a man who was almost a foot smaller and double his age.
But his claims were rejected and he was found guilty of culpable homicide amid highly-emotional scenes at the High Court in Glasgow.
A large throng of Mr Morrison family, including his partner of five years David Evans, were seen sobbing with relief as the killer was convicted. One said as she left the courtroom: "That's justice for John."
Murphy's bail was continued until sentencing in January but he is expected to be jailed for at least five years.
Mr Morrison was attacked in the early hours of December 22 last year. He had 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 David."
The court was shown CCTV of footage of Mr Morrison in Argyle Street, close to the St Enoch Centre.
Murphy and his friends were on the same area heading to the Arches Nightclub. Defence witness Samantha Mulgrew claimed that Mr Morrison, of Milngavie, suddenly appeared next to her mumbling, which gave her a "fright".
Murphy and his group then turned on the solictor. CCTV showed the mob heading towards him before one pushed Mr Morrison as he appeared to be walking away.
Seconds later - and out of shot - Murphy then 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. The ferocity of the blow had also broken his eye socket and nose.
Murphy's friend Miss Mulgrew later told police that he had landed a "proper punch" on the ex-council leader.
But, during the trial, Murphy, of the city's Bridgeton district, 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.
His QC Donald Findlay asked: "There was an implication that this was you being cowardly...that a fair reflection of events?" Murphy denied that.
Murphy then claimed he did not know Mr Morrison was seriously hurt at the time, but when he later discovered about the death he "felt sick".
Prosecutor Paul Kearney said the killer had lied during evidence, adding: "You went back to sort out Mr Morrison and punched him."
Murphy replied: "No."
The court also heard Murphy demanded friend Graeme Thomson not to say to police he had punched the lawyer. Murphy told the jury: "Those words did not cross my mouth."
Murphy will sentenced next month after Judge Norman Ritchie QC adjourned for reports.
But he told Murphy: "May I say at this stage that it is plain your character before this was a good one - but I have not been impressed with your conduct since this time.
"Your friends appeared to have tried to cover up for you and your lack of frankness continued in your evidence."