Astor, who was suffering from Alzheimer's disease, was 105 when she died in 2007.
During his trial, prosecutors said Marshall stole from his mother by taking expensive items out of her home or by getting her to change her will.
Marshall didn't testify at his trial, but his attorneys argued that he had the legal authority to give himself gifts with his mother's money.
Jurors found him guilty of grand larceny and scheming to defraud.
Marshall was sentenced to more than three years in prison, but appeals delayed him from serving any time until 2013.
A couple of months later, he was granted medical parole.
Marshall's misdeeds came to the public's attention in 2006 after one of his sons publicly accused Marshall of looting his mother's money.
Marshall was a decorated Marine, who was wounded at Iwo Jima during World War II.
He served as an ambassador to Kenya, Madagascar and Trinidad and Tobago.
Marshall was also an author and Broadway producer.
Attorney Kenneth Warner says Marshall died Sunday at New York-Presbyterian Hospital. He was 90.