Brooke Astor‘s 85-year-old son was convicted Thursday of exploiting his philanthropist mother’s failing mind and helping himself to her nearly $200 million fortune.

Anthony Marshall now faces a mandatory jail sentence of at least one year — and perhaps as many as 25 years — after jurors delivered their verdict to end a five-month trial that revealed the New York society doyenne’s sad decline. She was 105 and had Alzheimer’s disease when she died in 2007.