Van Houten, 72, “currently poses an unreasonable danger to society if released from prison at this time," Mr Newsom said in his parole review. It was the fifth time that a California governor has rejected her release.
Her attorney, Rich Pfeiffer, disputed that view and said the decision will be appealed in court. He accused Mr Newsom of rejecting parole because he is worried about “his political future" and noted that Van Houten has a spotless prison disciplinary record.
“We're not fighting (over) Leslie being a good person. She's proven that through her actions for half a century," he said.
Van Houten is serving a life sentence for helping Manson and others kill Los Angeles grocer Leno LaBianca and his wife, Rosemary, in August 1969. Van Houten was 19 when she and other cult members fatally stabbed the LaBiancas and smeared the couple’s blood on the walls.
The day before, other Manson followers, not including Van Houten, killed pregnant actress Sharon Tate and four others.
In his rejection letter, Mr Newsom noted that Van Houten had undergone therapy, earned educational degrees and taken self-help classes in prison and had shown “increased maturity and rehabilitation."
But Van Houten also has “gaps in insight" that continue to make her a danger to society, Newsom said.
Van Houten has had 21 parole hearings since 1982. Most parole boards denied her bid for freedom. But five panels have recommended her release since 2016, saying she had expressed remorse and was no longer a threat to public safety.
Newsom rejected a recommendation made last November.
He previously reversed parole recommendations in 2019 and 2020. In February, the California Supreme Court refused to hear Van Houten's appeal of the 2020 rejection.
Previous Gov Jerry Brown rejected Van Houten's parole in 2016 and 2018.
Manson died in 2017 of natural causes at a California hospital while serving a life sentence.