Jonathan Jones, 34, secretly drove 200 miles from his home in Orpington, Kent, to the isolated South Wales farmhouse of Harry and Megan Tooze where he killed them with a shotgun.
He then returned to the flat he shared with their only daughter, Cheryl, also 34, pretending he had spent the day looking for rented offices to set up a market research firm.
Police found the bloodstained bodies of the Toozes under hay bales in a cowshed at their smallholding at Llanharry, Mid-Glamorgan. Jones hid his secret, attending a news conference where Cheryl made a television appeal for information to help catch the killer.
He was arrested and charged five months after the murders although police could find no forensic evidence for the prosecution case. After a 12-week trial at Newport Crown Court, Gwent, Jones was found guilty of by majority 10-2 verdicts after the jury spent two days deliberating the case.
Sentencing him to concurrent life sentences, Mr Justice Rougier said Jones had been found guilty of the "ruthless and terrible" killings of the Toozes in July 1993.
Jones shook his head as he was led away. Cheryl Tooze was not in court for the verdicts but she has stood by Jones since his arrest five months after the deaths of her parents shocked the close-knit Welsh farming community.
The court heard that Jones and Miss Tooze were regularly in arrears with their rent and the ambitious Jones was planning to set up a market research firm although he had less than £100 in his bank account. He knew his bride-to-be was the sole beneficiary of her parents' estate.
Mr Tooze, 64, and his wife, 67, were both shot in the head from close range and died instantly. Neighbours who heard shots thought Mr Tooze, a retired fruit wholesaler, was shooting rabbits. Jones first shared a cup of tea with his victims, leaving his left thumbprint on a cup and saucer. But police found no evidence from his clothes or car to link him directly to the crime. The murder weapon was never found. No estate agents in Orpington, where Jones said he was looking for premises, remembered seeing him on the day of the murders.
Giving evidence during the trial, Miss Tooze said she did not believe her fiance had killed her parents. She and Jones had been together for eight years since meeting on a business studies course at the Polytechnic of Wales.
The jury was told there was several hours' delay in sealing off the scene of the crime because police first suspected suicide rather than murder. Jones gave evidence in his defence insisting his innocence and claiming he was "like a son" to Harry Tooze.