Darren Murphy hit John Morrison, 50, in the face during a motiveless attack in Glasgow city centre last December.
Mr Morrison - who had been enjoying a work night out shortly before the assault - died almost instantly after striking his head on the ground.
Bank worker Murphy left his victim in the street to go to a nightclub with friends.
Murphy told a jury he had been acting in self defence, against a man who was almost a foot shorter than him and double his age.
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.
A large throng of Mr Morrison's 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.
Judge Norman Ritchie, QC, adjourned for reports and 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."
Mr Morrison - the former Liberal Democrat 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 David."
The court was shown CCTV of footage of Mr Morrison later in the city's Argyle Street, close to the St Enoch Centre.
Murphy and his friends were in the same area heading to the Arches nightclub.
Friend Samantha Mulgrew claimed during the trial that Mr Morrison, of Milngavie, 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 them heading towards him before one pushed Mr Morrison as he appeared to be walking away.
Seconds later - and out of shot - Murphy then punched Mr Morrison in the face. He then carried on to the nightclub.
During the trial, Murphy, of the city's Bridgeton, denied he had punched Mr Morrison - instead claiming he had "hit him with a palm" to get him away.
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".