Leslie Van Houten was just 19 when she took part in the 1969 stabbing murders of Rosemary and Leno LaBianca, a day after other members of the Manson family had killed pregnant actress Sharon Tate - wife of film director Roman Polanski - and four friends.
They scrawled messages on the wall in the victims' blood and Manson said he was trying to start a race war.
But Van Houten's lawyer says she is now "a beautiful, decent human being".
California's Board of Prison Terms will consider a parole request by Van Houten for the 14th time on Friday.
Favourable ruling
This time, a court has ruled that the board must consider her exemplary prison behaviour over the last 30 years and not just the nature of her crime.
But prosecutors say Van Houten, 52 and a former homecoming queen, must continue to serve her life sentence in the California Institute for Women in Corona.
State attorney Stephen Kay said: "This is not a garden variety murder case.
"The legislature in California has specifically said that the purpose of incarceration is punishment, not rehabilitation."
While in prison, Van Houten has earned a university degree, tutored inmates and led anti-drug programmes.
Her lawyer Christie Webb described her as "now the beautiful, decent human being that she would have been, had it not been for one night of violence".
'Unreasonable risk'
But Mr Kay, who has been involved in the Manson murders case since 1970 and who will again present the state's case against parole, said Van Houten "presents an unreasonable risk of harm to society".
Even if the parole board does want to grant her freedom, California's Governor Gray Davis could still order her kept behind bars.
Mr Kay said of the "tough-on-crime" governor: "I know how he feels about murderers and I think there is absolutely no chance he would allow Leslie Van Houten to be released from prison."
Manson himself, now 67, was denied parole for a 10th time in April, refusing to even attend the hearing because he did not want to be shackled.
His record has hardly been model. It includes 17 serious infractions in the last five years, including threats to kill prison guards and multiple incidents in which he hit, spit or threw coffee on guards and other prison employees.