Sixteen-year-old Lynn Siddons was stabbed 43 times and strangled during a canalside walk near her Derby home with her childhood friend Fitzroy Brookes in 1978.
Fitzroy, who was 15 at the time, was tried for the murder but later cleared after he changed his guilty plea and blamed his stepfather Michael for her killing.
Yesterday, after a six-week trial and a nearly nine hours of deliberation, an Old Bailey jury returned a 10-2 verdict finding Michael Brookes, 51, guilty of the murder his stepson had been accused of nearly two decades before.
Lynn's grandmother Flo, 81, had spearheaded a campaign through the civil courts and eventually to the criminal courts. She said she was delighted. "At last after all these years we have justice. That is all ever we wanted for Lynn."
Lynn's aunt, Cynthia Smith, said: "I don't know how I feel - it hasn't sunk in yet. I just wish Lynn was here with us today."
The two women have appeared in court every day with Lynn's mother, Gail - too upset to speak today - and heard how Michael Brookes "initiated and instigated" the murder.
Fitzroy Brookes, who was the chief prosecution witness in the trial, admitted stabbing the girl half a dozen times on the orders of his stepfather. But the ex-lorry driver had given the fatal strokes and immersed her head in water before dumping her body in bushes, he said.
Brookes' solicitor said he planned to appeal against his conviction. Peter Kilty said: "My client maintains his innocence. We are frankly surprised by the verdict. The judge emphasised time and time again in his summing up the extraordinary flaws in the evidence and how cautious the jury should be in accepting it."
Brookes' counsel, Jonathan Goldberg QC, told how the defendant had served a sentence of his own since the killing. He had moved 14 times in 18 years because of harassment from the media and his victim's family. Now he was a recluse, suffering deep anxiety and panic attacks.
He spent his day alone watching television and videos - too afraid to leave the house for fear of reprisals, enduring a "friendless and nomadic existence", Mr Goldberg said.
The Chief Constable of Derbyshire Police, John Newing, said he was personally pleased a decision had been reached.
"I would like to pay tribute to Lynn's family. Her grandmother Flo's courage and commitment have been remarkable. Without her, it is unlikely this matter would have been brought before an Old Bailey jury. Justice has finally been done. The tragedy is that nothing is going to bring Lynn back."Reuse content