Fromme, a follower of serial killer Charles Manson, finished serving her sentence for the 1975 assassination attempt in July 2008, but served additional time for a 1987 prison escape, ABCNews.com reported.
"Lynette Fromme was released about 8 a.m. today," prison spokeswoman Maria Douglas said. She completed her sentence at the Federal Medical Center Carswell in Fort Worth, Texas.
Fromme aimed a semi-automatic .45-caliber pistol at Ford on Sept. 5, 1975. The gun had four bullets, but none was chambered, and an alert Secret Service agent grabbed the gun from her. At the time of the assassination attempt, Manson and several followers were serving life sentences for the so-called Tate-LaBianca murders, in which Manson followers killed actress Sharon Tate and six others in the actress's California home one night and followed that bloodbath the next night by killing socialite Leno LaBianca and his wife.
Fromme's trial lawyer, John Virga, told ABCNews.com he did not know where she would go once released from prison.
Vincent Bugliosi, the Los Angeles County prosecutor who convicted Manson and his followers in the Tate-LaBianca case, said Fromme should not be released from prison.
"If you do something like that against the president of the United States, the need for deterrence increases," Bugliosi told ABCNews.com.
Fromme initially was incarcerated in a California prison, but was transferred to the federal women's prison in Alderson, W.Va., after attacking an inmate. While in the Alderson facility, Fromme escaped to try to visit Manson after becoming distraught at the news he had testicular cancer. She was captured two days later and then incarcerated at the Texas institution.