Marshall faces a mandatory jail sentence of one year and up to 25 years in prison.
The jury convicted Marshall on 14 counts, including first-degree grand larceny and scheming to defraud, but acquitted him on two charges, falsifying business records and another first-degree grand larceny count.
Marshall's co-defendant, estate lawyer Francis X. Morrissey Jr., was convicted on all five charges, including scheming to defraud, conspiracy and forgery.
Morrisey faces 7 years in prison.
"I'm stunned by the verdict," said Marshall's attorney, Frederick Hafetz. "We are greatly disappointed in it, and we will definitely appeal."
They will remain free on bail until their Dec. 8 sentencing.
Lawyers had said Astor was lucid when she wrote her final will. Astor was 105-years-old when she died, and was diagnosed with Alzheimer's . Prosecutors called many witnesses who testified Astor suffered from mental confusion the last years of her life.
Prosecutors said Marshall couldn't wait for his mother to die. They mentioned his purchase of a $920,000 yacht with her money.
After the jury left the courtroom, Marshall's wife, Charlene, stood at the rail with her hand on Marshall's shoulder, her eyes glistening. When reporters asked her for a response, she said only, "I love my husband," and gave him a brief hug. The couple walked out of the courthouse, hand in hand, to a waiting limousine.
Astor's third husband, Vincent Astor, was the son of multimillionaire John Jacob Astor IV. She took charge of her husband's philanthropic work after his death in 1959. Her efforts won her a Presidential Medal of Freedom, the nation's highest civilian honor, in 1998.
Marshall is her son from a previous marriage to stockbroker Charles "Buddie" Marshall, who died in 1952.
A civil case is pending until the conclusion of the criminal trial.
The Associated Press contributed to this report.
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