Stuart Humphreys attacked Scott Sheard, of Formby in Merseyside, outside a bar in nearby Southport in October 2009.
The then 24-year-old never fully recovered from his injuries and died following complications in 2012.
Humphreys, 38, of Kingfield Road in Liverpool, will be sentenced on 19 May at Liverpool Crown Court.
Initially, no charges were brought after Mr Sheard died but after a campaign by his family the case was reviewed.
The court heard how an argument began when Humphreys pushed Mr Sheard out of the bar, after he had tried to take a bottle of beer outside.
Humphreys claimed he acted in self-defence, saying Mr Sheard and another man, Luke Rigsby, threatened to stab him, although he admitted he did not see a knife.
He said he feared he was about to be punched when he launched his attack.
But CCTV footage of the fatal blow proved Mr Sheard was not acting aggressively, the prosecution said.
One witness described the punch as "good" and "fast", adding how Mr Sheard was unconscious before he hit the ground.
The Crown Prosecution Service (CPS) initially declined to charge Humphreys with assault because of insufficient evidence.
Following a review after Mr Sheard's death, it was again decided that no charges should be pressed.
After Mr Sheard's family took their campaign to the Victims' Commissioner for England and Wales, Baroness Newlove, the CPS decided to review the case.
Mr Sheard's mother, Fenella Hill, broke down in tears as the verdict was delivered.
Outside court, she said: "Justice had finally been done. This is what we've been fighting for."
She added the last six years had "been hell".
"Poor Scott - the first two-and-a-half years seeing him suffering - the poor boy should never have gone through that."
Claire Lindley, from the CPS, said: "There are no winners in this tragic case. Despite today's decision, Mr Sheard's family still have to try and come to terms with the loss of their son and our thoughts remain with them."
A nightclub bouncer has been found guilty of the manslaughter of a man who died more than two years after he felled him with a single punch.