Moore, 77, had served 32 years of a life sentence when she was released from the federal prison in Dublin, east of San Francisco, the Federal Bureau of Prisons said.
Bureau spokeswoman Felicia Ponce said she had no details on why Moore was released.
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 and pulled the trigger, Oliver Sipple, a disabled former Marine standing next to her, pushed up her arm. The bullet flew over Ford's head by several feet.
In recent interviews, Moore said she regretted her actions, saying she was blinded by her radical political views.
"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.
Just 17 days before Moore tried to kill the president, Ford survived an attempt on his life in Sacramento by Lynette "Squeaky" Fromme, a follower of Charles Manson.
Moore said that she was convinced at the time that the government had declared war on the left.
"I was functioning, I think, purely on adrenaline 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.
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."
Ford insisted the two attempts on his life shouldn't prevent him from having contact with the American people.
"If we can't have the opportunity of talking with one another, seeing one another, shaking hands with one another, something has gone wrong in our society. I think it's important that we as a people don't capitulate to the wrong element," he said.
Ford died just over a year ago.
Moore was born Sara Jane Kahn in Charleston, W.Va. She acted in high school plays and dreamed of being a film actress.
In the 1970s, Moore began working for People in Need, a free food program established by millionaire Randolph Hearst in exchange for the return for his daughter Patty, who was kidnapped by the Symbionese Liberation Army in 1974.
Moore 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.
She has said she fired at Ford because she thought she would be killed once it was disclosed that she was an FBI informant. The bureau ended its relationship with her about four months before the assassination attempt.
"I was going to go down anyway," she said in a 1982 interview with the San Jose Mercury News. "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.