Moore, 77, had served about 30 years of a life sentence when she was released from the federal prison in Dublin, east of San Francisco, said Felicia Ponce, a spokeswoman with the Federal Bureau of Prisons.
Ponce did not know the details of Moore's release.
Moore was 40 feet away from Ford outside a hotel in San Francisco when she fired a shot at him on Sept. 22, 1975. As she raised her .38-caliber revolver, Oliver Sipple, a disabled former Marine standing next to her, pushed up her arm as the gun went off, and the bullet flew over Ford's head by several feet.
In recent interviews, Moore said she regretted her actions.
"I am very glad I did not succeed. I know now that I was wrong to try," Moore said a year ago in an interview with KGO-TV.
Moore said she was blinded by her radical political views at the time, convinced that the government had declared war on the left.
"I was functioning, I think, purely on adrenalin and not thinking clearly. I have often said that I had put blinders on and I was only listening to what I wanted to hear," she told KGO.
Chronicle photographer Gary Fong captured Ford reaction
Moore's confusing background - which included five failed marriages, name changes and involvement with political groups like the Symbionese Liberation Army - baffled the public and even her own defense attorney during her trial.
"I never got a satisfactory answer from her as to why she did it," said retired federal public defender James F. Hewitt. "There was just bizarre stuff, and she would never tell anyone anything about her background."
Moore was born Sara Jane Kahn in Charleston, W.Va. She acted in high school plays and dreamed of being a film actress before going through a series of marriages, beginning with nuptials to Marine sergeant Wallace Elvin Anderson in 1949...
In the 1970s, Moore began working for People in Need, a free food ransom arrangement established by millionaire Randy Hearst in return for his daughter Patty, who was kidnapped by the Symbionese Liberation Army in 1974. She soon became involved with radical leftists, ex-convicts and other members of San Francisco's counterculture. At this time, Moore became an informant to the Federal Bureau of Investigation.
Moore has said she fired at Ford because she thought she would be killed once it was disclosed that she was an FBI informant. The agency ended its relationship with her about four months before the shooting.
"I was going to go down anyway," she said in a 1982 interview with the San Jose Mercury News. "And if I was going to go down, I was going to do it my way. If the government was going to kill me, I was going to make some kind of statement."
Moore was sent to a West Virginia women's prison in 1977. Two years later, she escaped, but was captured several hours later.