Stanley 'Tookie' Williams, 51, was executed by lethal injection at San Quentin Prison this morning.
The US Supreme Court rejected a last minute appeal and Governor Arnold Schwarzenegger denied clemency.
Thousands of people had gathered outside San Quentin prison in California ahead of the execution.
The former gang leader was convicted of killing four people in 1979 and sentenced to death. Williams had always maintained his innocence.
He founded the notorious Crips gang in Los Angeles and was found guilty by a jury of four murders relating to two armed robberies.
In the past 24 years his case had wound its way through various appeal mechanisms, all the way to the US Supreme Court, but to no avail.
His case had attracted so much attention because he claims to have undergone a reformation in prison.
He had written books renouncing the gang lifestyle and campaigned from his prison cell to turn young people away from crime.
Several Hollywood figures and the Reverend Jesse Jackson had taken up his cause.
In a statement issued last night Mr Schwarzenegger rejected Williams's claim of innocence and said he could find no justification for granting clemency.
