Fromme, now 60, left the Federal Medical Center Carswell in Fort Worth at about 8 a.m. Friday, spokeswoman Dr. Maria Douglas said in a statement.
Fromme, who got a life term, became the first person sentenced under a special federal law covering assaults on U.S. presidents, a statute enacted after the 1963 assassination of President John F. Kennedy.
Ford was walking to the California State Capitol from his hotel when Fromme pushed through the crowd, drew the pistol from a holster on her thigh and pointed it at the president as he shook hands with well-wishers. She was restrained by Secret Service agents who wrested the gun away from her and led the president to safety.
Fromme was granted parole in July 2008 and released "via good conduct time" after completing a 15-month sentence for unlawful escape from a federal correctional institution, according to the statement. That sentence was being served consecutively after a life sentence for threats against the president.
She escaped from a female prison in Alderson, W.Va., on Dec. 23, 1987, and was recaptured about two miles away on Christmas Day after a massive search. She was sentenced to an additional 15 months in prison for the escape. Fromme had said she escaped from prison to be closer to Manson.
It was unclear why Fromme was at Carswell, a facility that specializes in providing medical and mental health services to female offenders. A spokeswoman for the bureau of prisons did not immediately return a phone call Wednesday seeking comment.
"I knew someday she would be released," said John Virga, the Sacramento attorney who handled her trial.
Fromme served time in at least two other facilities before Carswell.
Manson is serving a life term in San Quentin in California for the 1969 murders of actress Sharon Tate and eight others. Fromme, one of his "family" of followers, was not implicated in those attacks.
Associated Press Writers Juan A. Lozano in Houston and John McFarland in Dallas contributed to this report.