The only son of Brooke Astor, the doyenne of New York society, was convicted last night of looting his mother’s $198 million estate.

Anthony Marshall, 85, the scion of one of America’s most famous families, was found guilty of swindling his mother as she battled Alzheimer’s disease before her death in 2007 at the age of 105. Mr Marshall, a former CIA agent and US ambassador turned Tony award-winning Broadway producer, was convicted of 14 of the 18 offences with which he had been charged, including fraud and first-degree grand larceny.

He listened calmly as the verdict was read. After the jury left the court, his wife, Charlene, her eyes glistening, gave him a hug and told reporters: “I love my husband.”

The white-haired Marshall, who suffered health problems during the five-month trial and once collapsed in a courthouse lavatory, now faces a mandatory jail term of at least a year when he is sentenced in December. The case rocked the “Silk Stocking District” on New York’s Upper East Side, where Mrs Astor was a revered figure. Among the 74 witnesses who gave evidence were her friends Henry Kissinger, the former US Secretary of State, the TV personality Barbara Walters and Annette de la Renta, the wife of the fashion designer Oscar.

Mr Marshall is Mrs Astor’s son by her second husband, Charles “Buddy” Marshall, who died in 1952. A year later she married Vincent Astor, heir to the real estate fortune of John Jacob Astor.

When Mr Astor died in 1959, Mrs Astor inherited his $60 million fortune, a $60 million family foundation and a 42-person staff, including an English butler and a French chef.

The socialite became a much-loved figure, giving away nearly $200 million to charity, including the Metropolitan Museum of Art and the New York Public Library.

The family scandal first hit the headlines in 2006 when Mrs Astor’s grandson, Philip Marshall, a university professor, went to court to accuse his father of “elder abuse”. Philip Marshall accused Anthony Marshall of making Mrs Astor live in squalor in her Park Avenue apartment, forcing her to “sleep in the TV room in torn nightgowns on a filthy couch that smells, probably from dog urine”.

Though the allegations were never substantiated, Anthony Marshall agreed to put his mother in the care of her best friend, Mrs de la Renta.

Prosecutors said that Anthony Marshall used his mother’s failing health to make her sign a series of documents amending her will and funnelling tens of millions of dollars in cash, property and artworks to him and Charlene, his third wife.

Anthony Marshall’s lawyer, Francis Morrissey, 66, was convicted of forgery charges. A prosecutor described the pair as “two reverse Robin Hoods who stole from charity in order to enrich themselves”.

The jury reached its verdict on its 12th day of deliberations after almost coming to blows.

The foreman sent a note to the judge earlier this week saying: “Due to heated argument, a juror feels personally threatened by comments made by another juror. With regards to her personal safety, she wishes to be dismissed anonymously.”

The judge kept the woman juror on the panel, providing the defence with possible grounds for appeal.

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