The women's old red Ford Fiesta car was compacted as part of the Government's scrappage scheme in 2008.
The court heard that the two women murdered Mr Banfield because they wanted to claim his share of the family home and pension for themselves.
He had "effectively signed his own death warrant" by announcing he that he wanted to leave them and was selling the house to start a new life in his native Trinidad in May 2001, the court heard.
Two days before he disappeared he told police his wife and daughter were trying to kill him, but asked them not to take any further action.
The pair, from Canterbury, Kent, are believed to have murdered him that night or during the weekend that followed.
Police believe the father of four may have been handcuffed to stop him struggling and smothered with a plastic bag slipped over his head. But the location of the body and how it was disposed of remains a mystery.
For years, his wife and daughter claimed his annuity, telling friends and family he had simply vanished as they pocketed more than £120,000 from the sale of the house.
Police treated his disappearance as a missing person case until 2009 when the investigation was reopened after his former employer William Hill became suspicious.
Mrs Banfield then hampered the police search by giving a false description of her husband.
The women told police Mr Banfield was a womaniser and gambler and often went off when he had money.
They claimed to have seen him in December 2008, but later admitted lying to cover up the fraud.
Mrs Banfield even told her son Kevin, now 43, his father was alive and she had been in contact with him.
But lies unravelled when police discovered the pensions fraud.
The family home in Wealdstone, north west London, was forensically examined in October 2009. The garden was dug up and the concrete floor in the garage was taken up, but there was no sign of a body.
The two women were arrested last year after inquiries in Britain, Trinidad and New York failed to find any proof that Mr Banfield was still alive.
Mr Banfield's mother Irene died in 2004 without knowing what had become of him.
The women pleaded guilty to charges involving plundering £64,000 from Mr Banfield's pension funds by pretending he was still alive.
But Mrs Banfield collapsed in the dock as the jury returned guilty murder verdicts by a majority of 10-1, after more than 14 hours of deliberation.
The retired tax inspector was told she would serve at least 18 years and her daughter at least 16 years.
Kay Hackett, Mr Banfield's sister, said in a statement to the court: "Years of not knowing where Don was and then discovering the people closest to him had done such a wicked thing was unimaginable.
"Most painful now is not knowing how he died or where his remains may be."
Speaking outside court, DCI Howard Groves said the women had created "a complicated web of deceit".
He said: "Shirley and Lynette Banfield convinced themselves they would never be found guilty of his murder, however today's verdict shatters that belief."