Former auctioneer’s clerk James Sootheran, known as Anthony, 59, was found dead in his bedroom at High Havens Farm in the village of South Newington, Oxfordshire, on March 18 2014.
Mr Sootheran, who owned the property, including 60 acres of land, was more than six foot tall but weighed less than nine stone, having lost so much weight due to malnutrition.
His death was not initially treated as suspicious, but prosecutors urged police to investigate after Lynda Rickard, 62, was found to have used his bank account after he died.
She had been paid £47,000-a-year to care for his elderly mother Mary Sootheran, known as Joy, until her death, aged 92, in 2012, and lived rent free at the farm with her American-born husband Wayne Rickard, 66, before they were evicted in 2017.
Mrs Rickard admitted helping herself to tens of thousands of pounds of the Sootherans’ money, which was used to fund her family’s lifestyle, including private school fees for her three now-adult children.
She also accepted forging the mother and son’s wills, which would have entitled her to half of Mrs Sootheran’s £1.5 million estate and a third of Mr Sootheran’s £3.5 million fortune.
Mrs Rickard did not give evidence at a trial at Reading Crown Court but claimed the death of Mr Sootheran – described as “a recluse prone to gross self-neglect” with “complex mental health issues” – was a result of how he chose to live his life.
But she was found guilty of murder following a retrial on Friday.
Her husband was acquitted of murder but found guilty of the lesser charge of causing or allowing the death of a vulnerable adult.
The pair were also both found guilty of fraud by false representation relating to a £30,000 Mitsubishi Shogun paid for with Mrs Sootheran’s money in 2010, while Mr Rickard was convicted of a charge of perverting the course of justice.
Mrs Rickard earlier pleaded guilty to four fraud charges, two counts of forgery, two counts of conspiracy to pervert the course of justice, and possession of articles for use in fraud.
The judge, Mr Justice Wall, remanded the couple in custody ahead of sentencing on Tuesday.
Senior Crown Prosecution Service (CPS) prosecutor Robbie Weber said: “This is a landmark case.
“We understand it is the first time in 100 years that a charge of murder by way of deliberate starvation and, in this case, a significant financial gain, has been brought.
“We were able to prove that Lynda Rickard deliberately starved Anthony to death in order to inherit from his £3.5 million estate, having forged his will.”
He added: “The medical evidence played a vital role in this case, helping to prove that Anthony had been neglected for some time and tragically starved to death.
“Anthony would’ve survived if he received medical treatment a week before he died when it would have been obvious to anyone caring for him that he was seriously ill.
“However, getting help was the last thing on Lynda Rickard’s mind. She needed Anthony to die so she and her family could benefit from his fortune.”
The Rickards, of Edinburgh Close, Banbury, stood trial alongside Mrs Rickard’s godson Michael Dunkley, 49, Denise Neal, 41, and Shanda Robinson, 51.
Dunkley, of Brickle Lane, Bloxham, and Neal, of Radway Road, Lower Tysoe, Warwickshire, were found guilty of one count of fraud by false representation between January 28 and May 31 2014 after falsely claiming a will in the name of Mr Sootheran was genuine.
Robinson, of Sage Road, Banbury, Oxfordshire, was found guilty of one count of fraud by false representation, after falsely claiming a will in the name of Mrs Sootheran was genuine, and conspiracy to pervert the course of justice.
June Alsford, 78, of Little Lane in Aynho, Northamptonshire, previously pleaded guilty to one count of fraud and one count of conspiracy to pervert the course of justice.