Mary McElroy was an American woman known as being the sister of the 21st president of the United States, Chester A. Arthur, for whom she served as a hostess for his administration (1881–1885). She assumed the role because Arthur's wife, Ellen, had died nearly two years earlier.

Ellen Lewis Arthur, known as Nell Arthur, was the wife of the 21st president of the United States, Chester A. Arthur. She died of pneumonia in January 1880; her husband was elected vice-president that November. He succeeded to the presidency in September 1881 when President James A. Garfield was assassinated.