Mary Arthur McElroy (July 5, 1841 - January 8, 1917) was the sister of the 21st President of the United States, Chester A. Arthur. She acted as the First Lady of the United States from 1881 to 1885 after the Presidents wife, Ellen Lewis Herndon Arthur, died 18 months earlier.
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.
She held many functions in the White House. When her brother retired as the President, Mary went to live in Albany, New York. She died at the age of 75 in Albany.