Mary Arthur McElroy (July 5, 1841 - January 8, 1917) was the sister of 21st President of the United States, Chester A. Arthur, and served as a hostess for his administration (1881-1885).

She assumed the role because Arthur's wife had died a year earlier. Mary Arthur was born in Greenwich, New York to William and Malvina S. Arthur. She was the last of nine children. She attended the Emma Williams Seminary in Troy, New York and later married John McElroy. The couple had two children.

In 1880, Mary's brother Chester was elected vice-president, but in July, 1881, president James Garfield was fatally wounded and died on September 19, 1881. Chester succeeded him, and without a wife, he had Mary act as first lady. During the six months of mourning for Garfield, Arthur had the White House renovated. When he and Mary moved back in after the renovations, Mary brought her children to the White House. It is said they were the best behaved children to ever live in the White House. When Arthur refused his party's nomination in 1884, Mary retired to Albany, New York where she died at the age of 75.