Kevin Hutchinson-Foster was convicted of passing the weapon to Mark Duggan, 29, just 15 minutes before Duggan was shot in the chest in the north London suburb Tottenham.
Local protests against Duggan's death amid rumours that he was unarmed swiftly escalated into a wave of violence across London and other major English cities that left five people dead.
The conviction of Hutchinson-Foster at London's main criminal court on Thursday, after an earlier trial failed to reach a verdict, boosts the police case that Duggan was armed, even if he did not open fire.
Initial reports that father-of-four Duggan had fired at officers were dismissed after tests found that a bullet that had lodged itself in one officer's radio was police issue.
The court heard that Duggan had gone in a taxi to east London to collect the gun, hidden in a shoebox, from Hutchinson-Foster, 30, on August 4, 2011.
Armed police in unmarked cars pulled over the taxi 15 minutes later and officers told the trial they shot him in the chest and arm after seeing Duggan get out holding a loaded gun.
The weapon was found nearby after the shooting.
Hutchinson-Foster will be sentenced at a later date. He had denied the charge of passing the weapon to Duggan but had earlier pleaded guilty to beating someone with the gun several days earlier.
Senior Scotland Yard officer Dean Haydon said an investigation by Britain's police watchdog into Duggan's death was still under way.
"The Kevin Hutchinson-Foster trial has primarily been about the supply of an illegal firearm and I welcome the verdict of the jury in this case today," Haydon said.
Duggan's family have repeatedly called for justice and said they have still not received a full explanation of how he died.
An inquest is due sometime this year.
Riots erupted in Tottenham shortly after Duggan's death and within days an orgy of violence, looting and arson spread across London and then into other parts of the country.Do you have any story leads, photos or videos?