Moore, 77, had been scheduled for release on Tuesday, but she was instead freed a day early.
Moore pleaded guilty to attempted assassination and was sentenced to life in prison. She was released after 30 years because she had not broken prison rules, said Tom Hutchison, chief of staff for the U.S. Parole Commission.
The mother of four children, Moore fired a single pistol shot at Ford on Sept. 22, 1975, as the president stood in front of the St. Francis Hotel, near Union Square. She missed, mostly because a bystander knocked her arm away as she fired.
Her attempt came less than three weeks after Lynette "Squeaky" Fromme pointed a loaded revolver at Ford in Sacramento. She was arrested before she could fire a shot.
Ford died of natural causes in 2006.
The Associated Press contributed to this story. Reach Matt Krupnick at 925-943-8246 or firstname.lastname@example.org.