The California Department of Corrections and Rehabilitation said Friday that Simon's body was found in his one-man cell shortly before midnight Thursday. The cause of death is unknown, and an autopsy is planned.
Simon was convicted with Larry Green, Manuel Moore and Jessie Lee Cooks of first-degree murder and conspiracy to commit first-degree murder after an 18-hour deliberation by the jury in 1976.
The 69-year-old was convicted of two first-degree murder charges in 1976 and was serving a life sentence with the possibility of parole. The three others are serving life sentences.