Her case had become a cause celebre for women's groups because Thornton, 41, said that her husband, Malcolm, beat her up regularly. It put the issue of reforming the law on domestic violence firmly on the political agenda.
Thornton said after yesterday's verdict: "I am too tired to feel a sense of victory and there has been too much pain and at the end of the day, Malcolm died."
But the verdict after a 12-day retrial at Oxford Crown Court, ordered by the Court of Appeal, did nothing to resolve the issue of how the courts should deal with women driven to kill by repeated domestic violence.
Mr Justice Scott Baker sentenced Thornton to five years imprisonment for manslaughter, but said that the outcome was the result of evidence that she was suffering from a severe personality disorder rather than a question of provocation.
He told Thornton "I sentence you on the basis that your responsibility for killing your husband was diminished by your abnormality of mind."
Thornton showed no emotion as the jury of eight men and four women returned their verdict after deliberating for six hours and staying overnight in a hotel. She silently mouthed "I love you" to her daughter Luise moments before the jury foreman announced its decision.
Luise,18, and Barbara Garver, Thornton's sister, wept with relief at the verdict.
Across the court Malcolm Thornton's family looked shocked and disappointed by the decision. Gladys Sothers, his sister, burst into tears.
The judge's sentence meant that Thornton could walk free as she had already served five and a half years of a life sentence imposed in 1990 when she was convicted at her first trial of murdering her husband.
"I do not think that you represent a continuing danger to the public and the sentence I am going to pass will not mean that you have to return to prison," the judge told her.
Thornton never denied killing her husband but claimed that she stabbed him accidentally after a row as he lay drunk on the sofa of their home in Atherstone, Warwickshire, in 1989. The prosecution claimed that she was a "pathological liar" who killed him for financial reasons.
She lost her first appeal but what had been a domestic murder case which had passed largely unnoticed was taken up by women's groups campaigning for a change in the way courts deal with domestic violence cases.
A high-profile campaign followed which culminated in a second appeal hearing last December, at which her lawyers said that she was a victim of "battered woman syndrome" as a result of her husband's repeated violence, which caused her to lose control and kill him.
The Court of Appeal quashed the murder conviction and ordered a retrial. But the question of battered woman syndrome played little part in her second trial, during which much attention was paid to her personality disorder.
Psychiatrists told the court that Thornton suffers from a condition called dissociation, which causes her to react inappropriately to events and tell people what she thinks they want to hear.
After the verdict yesterday, Mrs Sothers said: "We basically think that the jury has bowed to feminist pressure. No one could have stood up to that."
Jean Murray, another of Mr Thornton's sisters, added: "It has cleared Malcolm's name in that she has not proved that she was a battered wife. She has just proved that she has an abnormality of mind."
But Thornton said later: "We don't know how the jury found for manslaughter, whether it was for provocation or for diminished responsibility. They did come back and ask questions on provocation and so obviously the issue was uppermost in their minds."
She said that she thought the verdict and the sentence were fair and added: "I am not saying that every woman should be sent to prison, but for me it was fair. I took a life at the end of the day."
As for her future, Thornton said she planned to write a book. "I have a vision of prisons as places of history rather than punishment. Prison was a healing place for me." She added: "I have a new life to build."
Asked about her conscience, she said: "I'm very, very judgemental of myself, probably more than anybody else. Forgive myself? Not yet."