Herman Wallace's attorneys said he died Friday at a supporter's home in New Orleans. Wallace had been diagnosed with terminal liver cancer and stopped receiving treatment.
US district Judge Brian Jackson had ordered Wallace released from the Louisiana state penitentiary at Angola on Tuesday after granting him a new trial. Jackson ruled women were unconstitutionally excluded from the grand jury that indicted Wallace in the stabbing death of a 23-year-old prison guard, Brent Miller.
Wallace and two other inmates convicted in the guard's death came to be known as the "Angola Three".
Wallace, of New Orleans, was serving a 50-year armed robbery sentence when Miller was fatally stabbed in 1972. Wallace and the two others convicted in Miller's death were moved to isolation at the Louisiana state penitentiary in Angola. In 2009, Wallace was moved to "closed-cell restriction" at Hunt correctional in St Gabriel and recently was taken to the prison's hospital unit.
Amnesty International USA last year delivered a petition to Louisiana governor Bobby Jindal's office, containing 65,000 signatures calling the men's solitary confinement inhumane and degrading.
Wallace's lawyers said he asked that, after his death, they continue to press the lawsuit challenging Wallace's "unconstitutional confinement in solitary confinement for four decades".
"It is Mr Wallace's hope that this litigation will help ensure that others, including his lifelong friend and fellow Angola Three member, Albert Woodfox, do not continue to suffer such cruel and unusual confinement even after Mr Wallace is gone," his legal team said in a written statement.
Woodfox, who remains in custody, and Wallace have continued to deny involvement in Miller's killing and say they were targeted because they helped establish a prison chapter of the Black Panther party at the Angola prison in 1971, set up demonstrations and organised strikes for better conditions.
The third man, Robert King, was released after 29 years in solitary confinement.
