Julie Wass, 48, drove Torbjorn Kettlewell to the horrific stabbing, then sped him away to a woodland hideout they had checked out earlier in the day.
He stabbed Kelly Franklin - the mother of his three children and Wass's next door neighbour - more than 30 times in a brutal, jealousy-fuelled attack near her home in Hartlepool in August.
Both Kettlewell, of Oval Grange, Hartlepool, and Wass, of Kipling Road, Hartlepool, denied murder.
Kettlewell, 30, was convicted of murder and Wass was cleared of murder but found guilty of the alternative charge of manslaughter following a trial at Teesside Crown Court.
Having helped Kettlewell get away, Wass returned to the murder scene and spoke to a police officer at the cordon, and in a conversation captured on his body-worn camera, explained she knew the killer and victim, but did not say she knew exactly how the knife-man had fled.
CCTV images showed Ms Franklin walking in the area before the attack, not knowing Kettlewell was scouring local shops for her, with Wass nearby in her red Ford Focus.
Kettlewell, 30, was a narcissist, obsessed with knives and changed his name from Ian to Torbjorn in honour of a character from the videogame Overwatch.
He had previously threatened to kill Ms Franklin and the social worker he blamed for them losing care of their children following an incident in which he accidentally shot one of them with an air weapon.
Ms Franklin, 29, was trying to break free from his domineering behaviour, move on from their failed relationship and win back custody of the children and the court heard he feared losing her forever when he killed her.
He admitted stabbing her but claimed diminished responsibility due to his mental illness. Two psychiatrists gave evidence to say his personality traits were not severe enough to give him that defence.
Wass, a married mother of four daughters who worked for the local NHS trust, had a sexual affair with Kettlewell and would frequently take him shopping and give him lifts.
She was with him for much of the day of the murder, knew he was brooding about his ex-partner, helped him to find her and drove him away from the scene, the prosecution claimed.
Richard Wright QC, defending him, said no-one knew why Kettlewell lost control that day, and no-one expected he would physically harm anyone.
John Elvidge QC, defending Wass, said there was no evidence she knew of Kettlewell's plan, or that he had a knife that night, and she had nothing to gain from the murder.
Mr Justice Jacobs will sentence both on Wednesday morning after hearing a statement from the victim's family.