Sheila Keen-Warren, 61, was released 18 months after she pleaded guilty to second-degree murder for the shooting of Marlene Warren, Florida Department of Corrections records show.
The plea deal came shortly before her trial would have started.
Keen-Warren, who has maintained her innocence even after her plea, was sentenced to 12 years in prison.
But she had been in custody for seven years since her arrest in 2017, and Florida's law in 1990 allowed significant credit for good behaviour.
It had been expected she would be released in about two years.
"Sheila Keen-Warren will always be an admitted convicted murderer and will wear that stain for every day for the rest of her life," Palm Beach County State Attorney Dave Aronberg said in a statement Saturday.
Aronberg last year conceded that there were holes in the case, saying they were caused by the three decades it took to get it to trial, including the death of key witnesses.
Greg Rosenfeld, Keen-Warren's attorney, has said she only took the plea deal because she would be released in less than two years and had been facing a life sentence if convicted at trial.
"We are absolutely thrilled that Ms Keen-Warren has been released from prison and is returning to her family. As we've stated from the beginning, she did not commit this crime," he said on Saturday.
Marlene Warren's son, Joseph Ahrens, and his friends were at home when they said a person dressed as a clown rang the door bell.
He said that when his mother answered, the clown handed her some balloons.
After she responded, "How nice," the clown pulled a gun and shot her in the face before fleeing.
Palm Beach County sheriff's investigators had long suspected Keen-Warren in the slaying, but she wasn't arrested until 27 years later when they said improved DNA testing tied her to evidence found in the getaway car.
Rosenfeld has called that evidence weak.
At the time of the shooting, Keen-Warren was an employee of Marlene Warren's husband, Michael, at his used car lot.
Since 2002, she has been his wife — they eventually moved to Abington, Virginia, where they ran a restaurant just across the Tennessee border.
Witnesses told investigators in 1990 that the then-Sheila Keen and Michael Warren were having an affair, though both denied it.
Over the years, detectives said, costume shop employees identified Sheila Warren as the woman who had bought a clown suit a few days before the killing.
And one of the two balloons — a silver one that read, "You're the Greatest" — was sold at only one store, a Publix supermarket near Keen-Warren's home.
Employees told detectives a woman who looked like Keen-Warren had bought the balloons an hour before the shooting.
The presumed getaway car was found abandoned with orange, hair-like fibres inside.
The white Chrysler convertible had been reported stolen from Michael Warren's car lot a month before the shooting.
Keen-Warren and her then-husband repossessed cars for him.
Michael Warren was convicted in 1994 of grand theft, racketeering and odometer tampering.
He served almost four years in prison — a punishment his then-lawyers said was disproportionately long because of suspicions he was involved in his wife's death.
