It was the third time a California governor denied the release of 71-year-old Bruce Davis.
The parole board found in March that he was suitable for parole based on his conduct in prison - he earned a doctoral degree while in prison and ministers to other inmates - and other factors.
Brown lauded Davis for his efforts to improve himself. However, he wrote in his decision that the evidence shows Davis "currently poses an unreasonable danger to society if released from prison."
Davis was convicted with Manson and others in the 1969 murders of a musician and a stuntman.