He was ordered released under California's compassionate release law, which was amended last year and allows for incarcerated people to be freed if they have a serious and advanced illness with an end-of-life trajectory.
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Maniscalco was the co-founder of the Hessian Motorcycle Club. Prosecutors said Maniscalco thought Richard Rizzone, another Hessian, was ripping him off in a counterfeiting and meth distribution ring.
Rizzone, his 19-year-old girlfriend Rena Miley and his bodyguard Thomas Monahan were killed in the 1980 Memorial Day attack in Rizzone's home in Westminster, which is about 25 miles southeast of downtown Los Angeles. All three were shot multiple times at close range.
Thomas Maniscalco, 77, who killed three people in 1980, will be released this week following a terminal cancer diagnosis. (Fox News)
Prosecutors said Miley, a police officer's daughter, and Monahan were slain so the killers could avoid witnesses.
Maniscalco and a fellow Hessian were convicted in the massacre. A third biker was killed by police in Oklahoma before charges could be brought.
Orange County District Attorney Todd Spitzer said Maniscalco, who will be released to his daughter who lives in the San Francisco Bay Area, poses a threat to public safety.
"He has taken no responsibility for the lives he destroyed and soon he will be a free man with nothing to lose," Spitzer said in a news release.
The state corrections department declined to comment Thursday. It was not immediately clear whether Maniscalco had an attorney who could speak on his behalf.