Cleveland: A judge has dropped all charges against a man who has spent 39 years in prison for murder, making him the longest-held US prisoner to be exonerated, an attorney for the Ohio Innocence Project said.

Ricky Jackson, 57, will walk free on Friday after paperwork is completed, attorney Mark Godsey said.

Exonerated: Ricky Jackson will walk free on Friday. Credit:AP

Mr Jackson was convicted along with two others for the 1975 murder of Harold Franks, a Cleveland-area money order salesman, after 12-year-old Eddie Vernon testified he saw the attack, according to court documents.

Mr Vernon, now 53, recanted his testimony and told authorities he had never actually witnessed the crime. There was no other evidence linking Mr Jackson to the killing.