The 16-year-old, who cannot be named for legal reasons, hit Bradley Moore outside a McDonald's restaurant in Ashton-under-Lyne in July 2016.
Shortly after, the 43-year-old was struck by Jonathan Carter's car.
Carter, 44, was also found guilty of causing death by careless driving at Manchester Crown Court.
Senior prosecutor David Hartley said it was "unusual" for both the boy and Carter to be convicted, but each had "independently contributed" to Mr Moore's death.
He said the 43-year-old had been attacked by the boy and three of his friends outside the restaurant on the evening of 27 July 2016.
The four "threw numerous punches and kicks" at Mr Moore before the 16-year-old "struck him with one punch which caused him to fall into the road," Mr Hartley said.
They then fled, leaving Mr Moore on the street, "unconscious and at risk of further harm," he said.
"Moments later, Carter, who was having a text argument with his partner, left McDonald's, got in his car and drove away.
"He failed to see Mr Moore lying prone in the road and drove over him, causing catastrophic injuries."
He added that he hoped the verdicts would give Mr Moore's "loved ones some sense of closure and that justice has been served".
The boy and Carter are due to be sentenced at the court on 27 March.
The three other boys who pleaded guilty to affray at an earlier hearing will be sentenced at juvenile court.
A boy who punched a man and left him lying unconscious in the street, where he was killed by a careless driver, has been found guilty of his manslaughter.