Marshall, 89, had served just eight weeks of his 1-to-3-year prison term.
"We're gratified by the parole board's unanimous and compassionate decision," Kenneth E. Warner, an attorney for Marshall, told The New York Times in an email.
Marshall has Parkinson's disease and congestive heart failure. He could not walk, stand, clean or dress himself, and had swallowing issues, his attorneys said recently.
Marshall and Francis X. Morrissey Jr., an attorney for the Astor family, were convicted in 2009. Authorities said the two tricked Astor, who was over 100 and had Alzheimer's disease, into altering her will to give the money to Marshall. She died in 2007 at 105.
Marshall surrendered at a New York City court June 21, and eventually was placed in the state's Fishkill Correctional Facility.