David Venables, 89, tried to blame serial killer Fred West for Brenda Venables’ murder, but was convicted by a 10-2 majority verdict on Friday.
His legal team said West had links to the Worcestershire village of Kempsey, where Brenda disappeared in May 1982, during the trial.
They also claimed Mrs Venables may have left her marital home at Quaking House Farm and “either killed herself or met with or encountered someone who wished her harm".
But the jury of seven women and five men deliberated for almost 17 hours over four days before convicting Venables of murdering his wife on either May 3 or May 4, 1982.
David Venables arriving at Worcester Crown Court Jacob King
At the start of the trial, prosecutor Michael Burrows QC said Venables had “got away with murder” for nearly 40 years after dumping his wife in the septic tank close to Quaking House Farm.
Her skull and other bones were discovered during work to empty the underground chamber on July 12 2019, six years after Venables had sold the property for more than £460,000.
Dismissing Venables’ defence as preposterous, Mr Burrows told the start of the trial: “The truth, say the prosecution, is that it was David Venables who killed her.
“He wanted her out of the way – he wanted to resume his long-standing affair with another woman, Lorraine Styles.
“He knew about the septic tank in its secluded location. It was for him almost the perfect hiding place.
“It meant he didn’t have to travel and risk being seen making a suspicious journey around the time of her disappearance or risk being seen disposing of her body somewhere else.
“And, of course, even if someone did think to look inside the tank, her body would be hidden from view.
“And for nearly 40 years, it was the perfect place and he got away with murder.”
The jury heard Venables’ affair with Ms Styles started around 1967, and continued on and off.
Mr Burrows said that by 1981, Ms Styles had “doubts again about David Venables’ feelings for her”, but that the farm owner rekindled the extramarital affair over that Christmas and new year, months before his wife vanished.
Venables, described by one witness at the trial as a smartly-dressed “typical gentleman farmer”, told the jury he woke up on the morning of May 4, 1982 to find his wife, then aged 48, had disappeared.
He said he then searched surrounding lanes and a stretch of the nearby River Severn.
Following the murder, the court heard, Venables appeared calm to those who knew him.
He later sought an annulment of his marriage to Mrs Venables, who was described by relatives and friends in court as a kind, hospitable and friendly woman.
Worcester Crown Court was told the pensioner informed police after his arrest in 2019 that he believed West may have killed Brenda, who had been diagnosed with depression.
A female friend of someone Venables knew, the court heard, had told him that “Fred West picked her up in Worcester at a bus stop early one morning, and she managed to escape”.
Venables, of Elgar Drive, Kempsey, told investigating officers: “I wondered since whether he was responsible for picking her up and eventually disposing of her body.”
It was also submitted on behalf of Venables that West had worked emptying septic tanks and that Mrs Venables’ disappearance had been unfairly “ignored” during inquiries into the Gloucester builder following his arrest by Gloucestershire Police in 1994.
Venables was remanded in custody and will be sentenced next Wednesday.
Brenda Venables in the garden of Quaking House Farm, Kempsey, Worcestershire West Mercia Police
Brenda Venables beside a car while on holiday in Perthshire, Scotland Family handout