She was born in Greenwich, New York. Her parents were William and Malvina S. Arthur. Her parents had nine children, Mary being the last. In 1880, her brother Chester Arthur, became the Vice President of the United States. In July 1881, after the assassination of the President, James Garfield, Chester A. Arthur became the president of the United States and he requested his sister to become "First Lady" during his term in office. 