Atkins, who had battled brain cancer over the past year, died of natural causes shortly before midnight Thursday night at the Central California Women's facility in Chowchilla, a state official announced.
Convicted of eight murders, Atkins served 38 years of a life sentence for her part in a killing spree that terrorized Los Angeles and put her in prison for life, The Los Angeles Times said. She was denied parole 13 times and was the longest-serving prisoner among women currently held in California penitentiaries.
Atkins confessed to killing actress Sharon Tate, the pregnant wife of director Roman Polanski, who was hanged and stabbed 16 times. Tate's nearly full-term fetus died with her. Four others died in the nightmarish assault at Tate's Hollywood Hills home.
The next night, Atkins accompanied Manson and his followers who invaded the Los Feliz, Calif., home of Leno and Rosemary LaBianca and killed them.
"She was the scariest of the Manson girls," said Stephen Kay, who helped prosecute the case. "She was very violent."
Former chief prosecutor Vincent Bugliosi, who sought and won death sentences for Atkins, Manson and other followers, said Atkins "apparently made every effort to rehabilitate herself." In prison, Atkins embraced Christianity and apologized for her role in the crimes.
It was Atkins herself who broke open the case, bragging about the slayings to a prison cellmate while incarcerated in another incident. She supplied crucial evidence that led to her own indictment as well as those of the others.
The death sentences, illegal in California at the time, were commuted to life in prison.
Atkins also pleaded guilty to the murder of musician Gary Alan Hinman, who was killed in a dispute over money shortly before the Tate-LaBianca murders and received another life sentence.
Born Susan Denise Atkins in San Gabriel on May 7, 1948, she grew up in San Jose, the middle child of three. When she was 15, her mother died of cancer. Her father, who sold the family home and all their furnishings to pay the hospital bills. became an alcoholic and deserted Susan and her younger brother.
The petite, dark-haired teenager became a drifter and eventually, in 1967, landed in Haight-Ashbury, San Francisco's haven for hippies and other wanderers, and met Manson.
2014: The Year in Music [PHOTOS]