The 16-year-old denies manslaughter and says he acted in self-defence when he hurled himself at Vijaykumar Patel in January.
The 16-year-old defendant, who cannot be named for legal reasons, denied manslaughter in the week-long trial at the Old Bailey, stating he acted in self-defence.
Vijaykumar Patel, 49, suffered a "catastrophic" head injury outside the minimarket where he worked in Mill Hill, north London, in January.
The teenager from Brent, northwest London, entered the shop with two friends and attempted to buy Rizla papers.
When they were sent away for being under 18, the three became angered and threatened to vandalise the shop.
Mr Patel and a colleague went outside to ensure no damage was done to the property.
The defendant shouted abuse before hurling himself at Mr Patel outside the shop, prosecutor Louis Mably QC said.
He added "it is possible to see the moment Mr Patel was hit and which boy delivered the blow" as the attack was captured on CCTV.
"This boy suddenly lunged at Mr Patel, flew towards him and smashed him in the upper part of his body with his right forearm," Mr Mably told the jury.
"When the boy smashed into the lower part of his face it caused Mr Patel, with his hands in his pockets, to fall backwards onto the pavement and smash his head.
"It was that moment, that blow causing him to smash his head, that caused his death," he continued.
Mr Patel was found unconscious and bleeding in the street by police on 6 January and died the next day at St Mary's Hospital in central London.
His colleague was also punched by the teenagers but did not require medical treatment for his injuries.
The boys were "laughing, joking and happy about what had taken place" as they fled the scene, Mr Mably added.
The defendant was found guilty following the trial and sentencing was adjourned to 7 September.