Phillip Begley, 44 of Wellington Road, Bridlington, was found guilty of the manslaughter of Paul Morrison, 51, after a trial at Leeds Crown Court.
The jury heard Begley had punched Mr Morrison, causing him to fall back and hit his head on a pillar.
Mr Morrison died three weeks later in Harrogate Hospital.
Begley carried out the attack in the Wetherspoons Winter Gardens pub in Parliament Street on 1 November.
He was also found guilty and sentenced to six months for assaulting Peter Bell, who was drinking with Mr Morrison at the time.
Mr Bell suffered facial injuries in the attack, which required hospital treatment.
Det Chf Insp Pete Martin, of North Yorkshire Police, said he welcomed Begley's sentence.
"Paul Morrison was a devoted family man whose wife and children have been left heartbroken by the actions of Philip Begley," he said.
"Paul was enjoying a night out with his friends and he never came home, which is a devastating and tragic scenario which no family should ever have to deal with.
"At no time has Begley accepted responsibility for his actions and has forced Paul's family to sit through the anguish of a crown court trial."
A man has been jailed for six years after killing his work colleague with a single punch in a Harrogate pub beer garden.