Eyjolfur Andrews, 16, was convicted at the Old Bailey by the jury of six men and six women. He will be sentenced today.
He shot Amaranath Bandaratilleka, 32 - a barrister's clerk known to his friends as Nath Banda - at point-blank range with a sawn-off shotgun during a hold- up at a newspaper shop in King Street, Hammersmith, west London, last February. Mr Banda died in hospital.
Andrews admitted manslaughter, robbery and possession of a firearm.
These charges were denied by his friend Sam Perman, 18, of Ravens court Gardens, Hammersmith, who was with him at the time of the robbery. The jury found Perman guilty of robbery and possession of firearms. He was acquitted of the murder of Mr Banda, but found guilty of his manslaughter. Andrews had denied the murder charge.