Davis was convicted in the 1969 killings of Gary Hinman, an aspiring musician, and Donald "Shorty" Shea, a stuntman, but he did not participate in the infamous murder of Sharon Tate.
The state Board of Parole Hearings found Davis, now 70, suitable for parole last year.
In his decision today, the Democratic governor acknowledged Davis "has made efforts to improve himself while incarcerated," but he said the positive steps Davis has taken "are outweighed by negative factors that demonstrate he remains unsuitable for parole."
Brown, who uses his power to block decisions of the parole board relatively sparingly, called the killings of Hinman and Shea "heinous and brutal," and he said Davis played a central role. He also said Davis fails to acknowledge the extent of his involvement in the crime.
PHOTO CREDIT: A photograph of Bruce Davis taken in October 2012 / California Department of Corrections and Rehabilitation