Marshall, a decorated World War II veteran who later became a diplomat and Broadway producer, died Sunday morning at NewYork-Presbyterian Hospital, attorney Kenneth Warner said. Warner didn't immediately have information on the cause of Marshall's death, but Marshall had had heart and other health problems for years.
Marshall was convicted in October 2009 of taking advantage of his aged mother's slipping mind to loot her millions, and he was sentenced to one to three years in prison. Appeals delayed his incarceration for over 3 1/2 years, but Marshall ultimately went to prison in June 2013.
He was granted medical parole two months later because of debilitating illness, with a parole board asking whether he had regrets about the events leading to his imprisonment.
"Well, regrets, yeah," he said, "naturally."
Born into wealth, Marshall earned a Purple Heart in the battle of Iwo Jima and enjoyed a life of upper-class respectability that was shattered when one of his own sons, Philip Marshall, publicly accused him in 2006 of looting the society doyenne's money while letting her live in squalor. The allegations of physical neglect were never substantiated, but they led to the criminal case over Astor's finances.
Astor was 105 and suffering from Alzheimer's disease when she died in 2007. Years earlier, she was awarded the nation's highest civilian honour — the Presidential Medal of Freedom — for giving away nearly $200 million to charities and institutions.
Prosecutors said Marshall took advantage of her failing mind to help himself to her money.
His methods were as simple as taking artwork off his mother's walls and as complex as getting her to change her will to give him millions of dollars that had been destined for charity, prosecutors said during a five-month trial that featured testimony from such Astor friends as Barbara Walters and Henry Kissinger.
Marshall's lawyers said he had the legal power to give himself the raise and other gifts with his mother's money, and he believed she wanted him to have them. The defence lawyers also argued that Astor was lucid and acting out of love when she altered her will to benefit her son.
Jurors disagreed and found Marshall guilty of grand larceny and scheming to defraud.
Marshall didn't testify or call any witnesses in his defence. But after his conviction, he bared his personal life and enlisted such prominent supporters as Al Roker and Whoopi Goldberg in an effort to stay free.
He described in court papers an often sad, if privileged, upbringing as the only child of Astor. He painted his father — Astor's first husband, New Jersey state Sen. J. Dryden Kuser — as an alcoholic who pushed the pregnant Astor down a flight of stairs.
After they divorced, Astor married stockbroker Charles Marshall, who virtually banished her son to a series of boarding schools and summer camps, Anthony Marshall said. He took his stepfather's name nonetheless.
After Charles Marshall died, she married Vincent Astor, a scion of one of the country's first ultrarich families.
Meanwhile, Anthony Marshall joined the Marines out of high school, in the midst of World War II. He was wounded leading a platoon at Iwo Jima.
After the war, Marshall served as ambassador in posts including Kenya, Madagascar and Trinidad and Tobago. He wrote seven books on topics ranging from African art to U.S. zoos, and he co-produced Tony Award-winning runs of "Long Day's Journey Into Night" and "I Am My Own Wife." He was married several times.
"Tony had the tenderest of hearts, a brilliant mind and an outrageous sense of humour," his widow, Charlene, wrote in a death notice in The New York Times. She is not the mother of his twin sons, Philip and Alexander, and she stood staunchly by him during the criminal case.
While Marshall said he and his mother became close after Vincent Astor died in 1959, the trial depicted the relationship as difficult. His mother once told a friend, "I wish Tony had made something of himself instead of waiting for the money," according to testimony.
Marshall stepped aside as her guardian in 2006, after his son Philip filed court papers accusing him of neglecting her. Anthony Marshall denied any mistreatment.
The rift in the family remained even after his death.
"The fact I first heard the sad news of my father's death from the press speaks volumes about the devastating effect elder abuse can have on families," Philip Marshall said.
Reach Jennifer Peltz on Twitter @ jennpeltz.