After being convicted of financially exploiting his mother Brooke Astor, Marshall was sentenced to prison, though stayed there for only eight weeks in 2013 before receiving medical parole.

Known as Tony, Marshall was the only child of the American philanthropist Brooke Astor and her first husband, New Jersey state senator John Dryden Kuser.

After enlisting in 1942 during World War II, he served with the U.S. Marine Corps (his grandfather John H. Russell Jr. had served as commandant of the Marine Corps before the war) and led his platoon in the battle of Iwo Jima, attaining the rank of captain and earning a Purple Heart.

In the 1980s Marshall was an officer with United States Trust Company of New York where he assisted the bank with the management of large estate accounts.

[6] The couple were divorced on January 24, 1992, reportedly following Marshall's affair with Charlene Gilbert, the wife of an Episcopal priest in Northeast Harbor, Maine.

In July 2006 Philip Marshall filed suit against his father alleging mistreatment of his grandmother Brooke Astor and mismanagement of her funds.

On November 27, 2007, Marshall surrendered to authorities at the Manhattan district attorney's office to face indictment on sixteen counts relating to the handling of Brooke Astor's will and financial affairs.

He further said that "Marshall abused his power of attorney and convinced Mrs. Astor to sell property by falsely telling her that she was running out of money.

His health problems included Parkinson's disease and congestive heart failure; his lawyers claimed he could not walk, stand or dress himself.