In November 1880, McElroy's brother Chester Arthur was elected vice president. In July 1881, President James Garfield was fatally wounded and died on September 19, 1881. Arthur succeeded him, and asked McElroy to care for his young daughter Ellen and act as "Mistress of the White House." Because she had her own family in Albany, McElroy lived in Washington, D.C. only during the busy winter social season. Although Arthur never officially granted her the protocol of a formal position, she proved to be a popular and competent hostess. The procedures she and her brother developed for the social functions were used by future First Ladies for decades.