California Governor Jerry Brown has denied parole for Leslie Van Houten, a former follower of Charles Manson, who is serving a life sentence for murder.
A prison board had recommended parole.
However, Governor Brown said her "inability to explain her willing participation in such horrific violence" made him think she was still a risk to society.
Relatives of her victims opposed her release with a petition signed by 140,000 people.
Van Houten has now been denied parole 20 times over the 1969 killings of Leno La Bianca and his wife Rosemary.
Then 19, she held down Rosemary La Bianca while someone else stabbed her and she later admitted she stabbed the woman after she was dead.
Van Houten (right) was found guilty with other Manson Family members at a 1970 trial
"I don't let myself off the hook. I don't find parts in any of this that makes me feel the slightest bit good about myself," she said at her April parole board hearing.
Since her conviction, Van Houten, now 66, completed college degrees and demonstrated exemplary behaviour while in detention.
She was the youngest Manson follower to be convicted of murder.
She has been seen as the most sympathetic of cult leader's followers.
The La Bianca killings came a day after other followers of Charles Manson carried out the grisly murder of actress Sharon Tate and four others.
Prosecutors at the time said he controlled his followers using drugs and other means.
Manson, who directed but did not take part in the murders, thought the killings would start a race war, called "Helter Skelter" after a Beatles song.
Now 81, he remains in prison, as do others of his followers, including Patricia Krenwinkel and Charles Watson.