Peter Roberts, 55, died 18 months after Sam Whittet, 24, of Knuston Spinney in Irchester, punched him at Brafield-On-The Green Working Men's Club on December 10, 2016.
Whittet had already served a 33-month sentence for grievous bodily harm and two common assaults relating to the same incident but will now served an additional jail sentence after Peter died.
Whittet pleaded guilty to manslaughter on November 14, 2019.
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The court heard Whittet had been drinking at the club before the incident, and had consumed a minimum of six to eight pints and two jagerbombs.
In her sentencing remarks, Judge Adrienne Lucking said Whittet had been at the club with his girlfriend and was drinking at the bar while his brother attended an 18th birthday party being held at the club.
She said at some point in the evening Whittet's mood had changed and he was "spoiling for a fight".
Whittet argued with his brother and then went outside, where he punched a 15-year-old boy in an unprovoked attack that was caught on CCTV.
Judge Lucking said it was clear the punch was unprovoked and happened in front of young teenagers waiting to be collected from the party.
Whittet's girlfriend intervened and he walked away, which the Judge said showed he was at times in control of his actions.
Whittet then proceeded to punch and kick walls before fighting with his brother again. Their father arrived but was unable to calm Whittet down.
It was then that Peter Roberts, who had been at the club with his wife, Brenda, and daughter, Emma, approached Whittet.
Prosecutor Mary Prior QC described Mr Roberts as a "peacemaker".
She said: "He attempted to get the defendant to calm down. He was 5ft 9in and 53, the defendant was a taller and larger young man.
"He hit Peter Roberts with a left hook punch that was heard some distance away."
Mrs Prior said Whittet walked away from Mr Roberts as he lay on the floor and was detained by members of the public who sat on him until police arrived.
In her sentencing remarks, Judge Lucking said: "(Mr Roberts) went to intervene, telling you to calm down. Your response was to deliver a powerful punch."
She said Mr Roberts had been unable to put out his arms or save himself from the fall.
Mr Roberts suffered a fracture to the base of the skull and a bleed to the brain.
His family described in their victim impact statements how traumatic and difficult it was to see him in a coma for the next 18 months.
He was in a vegetative state with little to no awareness of himself or his environment.
While Mr Roberts was in a coma, Whittet was charged and pleaded guilty to grievous bodily harm and two common assaults - one of the 15-year-old boy and one for headbutting a police officer when he arrived in custody.
Whittet served a 33-month sentence and had been released from prison when Mr Roberts died from bronchopneumonia on July 31, 2018.
The court heard a medical report established that this was a result of his hospitalisation due to the traumatic brain injury inflicted by Whittet.
Mitigating, Greg Bull QC said Whittet was a hard-working young man who was used to drinking in large quantities but had never previously been violent.
Mr Bull said: "My client accepts he was drunk but finds it hard to understand how alcohol affected him to that affect on that night."
Judge Lucking said the offence was so serious that only a custodial sentence could be imposed.
Whittet had appeared emotional during parts of the sentencing but remained standing and looked at the judge calmly when she delivered the sentence.
After the sentencing hearing, Detective Sergeant Gavin Suttie said: “There is no sentence on this earth that can make up for the loss of Peter Roberts.
“He was a much-loved husband to Brenda and devoted father to Emma. Their anguish since this incident in 2016 simply cannot be put into words – every day is a struggle without Peter by their side.
“This has been an exceptionally long ordeal for them and I hope today’s sentencing brings them some element of closure, though their lives will of course, never be the same again.
“This has been a very complex investigation and I would like to thank my team for their diligence and professionalism throughout.
“I also hope this incident highlights just how extreme the consequences can be from a single punch and I hope Whittet continues to reflect on the devastation he has caused an entire family through completely mindless and avoidable actions.”