Van Houten was involved in the infamous 1969 murder of Los Angeles businessman Leno LaBianca and his wife at their home in the Hollywood Hills. The killing occurred one day after the attack that killed actress Sharon Tate and four others, which Van Houten was not involved with.
"While I commend Ms. Van Houten for her efforts at rehabilitation and acknowledge her youth at the time of the crimes, I am concerned by her role in these killings and her potential for future violence," Gavin said in a statement.
Van Houten earned her bachelor's and master's degrees while in prison, but Newsom said there are still too many negative factors surrounding her release -- and noted the 1969 deaths were some of the "most notorious and brutal killings in California history."
Manson was denied parole 12 times before he died in 2017 at age 83 while serving his life term.
In recommending parole for Van Houten, the board found her "an unreasonable danger to society."
"I take responsibility for the entire crime," Van Houten said at her last parole hearing in 2017. "I take responsibility going back to Manson being able to do what he did to all of us. I allowed it. I was easy to give over my belief system to someone else. I sought peer attention and acceptance more than I did my own foundation."
Van Houten is imprisoned at the California Institution for Women.