The convicted murderer walked out of a California facility Tuesday after successfully receiving parole earlier in the month ... this after Gov. Gavin Newsom said he would not challenge her release, despite doing so 3 different times throughout his tenure as Governor.

Van Houten was convicted of 2 counts of first-degree murder and one count of conspiracy to commit murder way back in 1971 for her involvement in the grisly 1969 LaBianca murders here in Los Angeles ... when Leno and Rosemary LaBianca were stabbed to death at their home. Van Houten said she held Rosemary down while someone else stabbed her, and later admitted she stabbed her as well after Rosemary was dead.

In the years since her conviction ... Van Houten's attorneys have said she was young and brainwashed by her cult leader, Charles Manson, at the time -- arguing she had been rehabilitated.

More importantly, Van Houten's lawyers have said she does not pose a threat to society anymore ... and it seems a handful of parole boards agreed with that, signing off on recommendations that she oughta be released, in spite of her life sentence.

Like we said, Newsom has shot her down time and again ... with his argument being, the nature of her crime should prevent her from ever enjoying a free life. But, after his last rejection ... she appealed his decision in court, and they ended up siding with her.

The governor's office said trying to fight it was a losing effort, so they were going to drop it.

Now, at age 73 ... Van Houten's back on the outside, and it'll be interesting to see what she does going forward. At first, she'll be sent to a halfway house where she'll learn basic skills and get up to date on all the new technology that's come out since she was imprisoned.

Van Houten is said to have been a constructive prisoner over the years, having found faith and showing an eagerness to teach/learn. She's also expressed remorse about her role in the murders.