Keir Hetherington was just 15 in January last year when he landed the blow on 41-year-old Darrius Skinderowicz outside the Euro Market store on Carlton Street.
Mr Skinderowicz, a Polish national, fell back and hit his head on the pavement. He was able to get back to his feet, but was found unconscious hours later outside the town's Poundstretcher store.
His life support machine was turned off at Pinderfields Hospital three days later after he suffered an inoperable subdural haematoma - a bleed to the brain.
Before the sentencing hearing this morning at Leeds Crown Court, the Recorder of Leeds, Judge Guy Kearl QC, agreed to lift reporting restrictions on the case, allowing Hetherington to be publicly named, despite being under 18, to help "deter others" from such behaviour.
The court heard how Mr Skinderowicz had been into the Euro Market store to buy a can of lager in the early evening of January 15, 2018, and was approached by a group of youths as he left.
Street surveillance CCTV played in court showed the group running towards him, before words are exchanged and two punches thrown by Hetherington. The first missed, but a left hand then connected causing the victim to fall back and strike his head.
Between the attack and Mr Skinderowicz collapsing later in the evening, he had to told his estranged wife that he had been attacked by youths because he had refused to go into the shop to buy them cigarettes.
After being arrested, Hetherington told police that he had confronted the victim because he said he had earlier taken money off one of his friends to buy cigarettes and kept the cash.
Hetherington admitted throwing the punches, but said he felt Mr Skinderowicz was about to attack him, which prosecutor Chloe Hudson claimed was an attempt to "lesson his role by acting in self defence".
Hetherington had also boasted to friends about 'dropping' a man, meaning he punched him to the ground.
Defending, Andrew Petterson, said his client had been "capable of acting mature" and had recently completed a Prince's Trust educational course.
He had also written a letter expressing his remorse and had admitted a charge of manslaughter at an early opportunity.
However, Recorder Kearl QC said 16-year-old Hetherington, of Aketon Road, Castleford, did not have an exemplary character and there had been incidents of assaults and bullying of fellow pupils at Castleford Academy.
Addressing Hetherington, Recorder Kearl QC said: "What is clear is that you led this group.
"We may have been told he had taken money from one of your group but it's difficult to see that has happened.
"You pursued him when he backed off and you swung two punches when he posed no threat whatsoever. You boasted to the group that were going to 'drop him' and that's exactly what you did.
"He was able to get up but the damage had already been done. It was an act to make you look big in front of others and to impress them."
He sentenced Hetherington to 26 months' jail, adding: "It's clear that no sentence can adequately replace the loss of a life."