Jennifer Johnson, 55, lied under oath to protect Russell Bishop – the killer of two nine-year-old girls in Brighton – at his first trial in 1987, Lewes Crown Court heard.
The lies she told helped to acquit him and three years after he walked free, Bishop struck again.
He committed another brutal sex attack on a third schoolgirl, leaving her for dead at Devil’s Dyke near Brighton in 1990.
Johnson, who had three children with Bishop, kept up her support for him until he was finally convicted of murdering friends Nicola Fellows and Karen Hadaway in 2018.
She only admitted her part in keeping Bishop out of prison when new scientific techniques exposed her lies.
Johnson tried to claim she was terrified of Bishop even after he was given a life sentence for the 1990 attack.
But the jury heard she could not remember the details of any specific threats which put her in fear for her life.
The prosecution’s case was that Johnson was more than capable of challenging her partner and making her own decisions.
At the trial, the jury were shown letters sent by Johnson to Bishop in 1990 while he was awaiting trial for the attack on a young girl, in which she talked of marriage.
Johnson initially told police a vital piece of evidence – a blue sweatshirt which was linked to the scene of the murders – belonged to Bishop.
In a dramatic courtroom reversal, she changed her story in the witness box saying he had never owned the sweatshirt with a Pinto motif.
Improved DNA tests conclusively linked Bishop to the shirt and he was given a second life sentence in 2019.
Johnson kept up her lies until she was told police wanted to speak to her about perjury.
Following a four-week trial at the same court where Bishop was tried twice, a jury found her guilty of perverting the course of justice and perjury.
Johnson closed her eyes as she heard the jury’s decision.
Mr Justice Peter Fraser will tell Johnson this afternoon if she is to be jailed immediately.
Sentencing is expected later this week.
The families of nine-year-old friends Karen Hadaway and Nicola Fellows were denied justice from 1986 to 2019 when Bishop was finally convicted at a double jeopardy retrial.
Their case became known as the Babes in the Wood after Bishop brutally murdered them and left their bodies in Wild Park, Brighton.
After the verdict, Libby Clark, Senior Crown Prosecutor, said: “Jennifer Johnson’s deception caused decades of distress to the families of Nicola Fellows and Karen Hadaway.
“It took a change in the law and more than 30 years for him to finally be convicted of their murders.
“She knew exactly how important her evidence was to the justice process, but still chose to put Bishop and herself first by lying at his original trial.
“Johnson claimed she had been acting under duress but this was just another lie.
“Her actions were clearly those of someone who wanted their partner acquitted of murder, rather than a person living in fear.
“Our thoughts are with the families of Karen and Nicola, who have had to wait far too long for justice as a result of the failure of Bishop and Johnson to accept responsibility for their crimes.”
The families of Karen Hadaway and Nicola Fellows were left devastated by the horrific murders and the aftermath.
Karen’s father Hadaway died from a heart attack in 1998 without seeing Bishop convicted of killing his daughter.
Nicola’s dad Barrie Fellows was dogged for years by entirely false claims by Johnson and Bishop implicating him in the murder of his own daughter.
His brother Kevin spent 18 months conducting his own investigation into the blue sweatshirt before he died of cancer without seeing justice for Nicola.
The seven-year-old girl Bishop attacked after he was acquitted was snatched off the street, thrown in a car boot.
Bishop drove her to Devil’s Dyke where he left her for dead.
She was spotted on the roadside, naked, freezing and terrified.
The girl was able to identify Bishop and he was jailed for life.