Ellen Lewis Arthur (August 30, 1837 – January 12, 1880), 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.

Nell's social network among the élite families of New York widened Chester's political contacts. The pair became a prominent pairing noted for their ambition to gain the recognition and prestige that would accompany Chester's rise in politics. Among Nell’s friends were Vanderbilts, Astors, and Roosevelts. Her mother's wealth allowed them luxuries such as the three-story Lexington Avenue brownstone townhouse with expensive furnishings from Tiffany’s, which Arthur could not have afforded by himself. Freed from the need to earn enough income to support the lifestyle the couple enjoyed, Arthur was able to devote himself to the New York Republican party, eventually rising through political patronage to Quartermaster General of the New York Militia, with the rank of brigadier general during the Civil War, and Collector of the Port of New York after the war.

On January 10, 1880, Nell Arthur came down with a cold. She quickly developed pneumonia and died two days later on January 12, 1880, at age 42 in New York City, New York. She was buried in the Arthur family plot in Albany, New York.

Arthur deeply mourned the death of his wife. After taking office as president, Arthur, who could see St. John's Episcopal Church from his office, commissioned a stained glass window dedicated to his wife at the church. He had it installed where he could view it at night, as the lights were kept on within the church. Additionally, he ordered fresh flowers placed daily before her portrait in the White House.

Mary McElroy (née Arthur; July 5, 1841 – January 8, 1917) was an American woman known as being the younger sister of the 21st president of the United States, Chester A. Arthur, for whom she served as a hostess (serving as the twenty-fifth first lady) for her older brother's administration (1881–1885). She assumed the role because Arthur's wife, Ellen, had died nearly two years earlier.

McElroy was born in New York and attended the progressive Emma Willard School. She married insurance salesman John Edward McElroy and worked for some time as a teacher. When her brother Chester A. Arthur became president, she lived in the White House during winter months to host social events and care for her niece. As acting first lady, McElroy ended the tradition of suspending social events during Lent, and she held large weekly receptions. She was a popular hostess, and contemporary Washington social life celebrated her for her lively receptions.

When her tenure as hostess ended, McElroy returned to her family in Albany, New York, and she later worked in Ireland to preserve the Arthur family homestead. She handled her brother's affairs during his illness and after his death, taking responsibility for his funeral and his presidential legacy. McElroy died in Albany in 1917. She is one of the most obscure first ladies, and relatively little scholarly research about her life exists.

Her brother Chester was elected vice president in 1880, and he became president after the death of President James A. Garfield in 1881. In January 1883, President Arthur asked her to serve as White House hostess. As Arthur was a widower, there had been no first lady in the White House for the first year of his presidency. Her close relationship with Arthur's daughter reaffirmed his decision to choose her as White House hostess. McElroy accepted the position, though she faced some reluctance, as she was a naturally shy person, and it would require her to leave her family in Albany. McElroy quickly learned the responsibilities of White House hostess, and she described herself when starting as "absolutely unfamiliar with the customs and formalities".

McElroy began her role as acting first lady on January 24 when she participated in a diplomatic corps dinner. McElroy first hosted a reception of her own on January 27 and would host further receptions on Saturdays thereafter. McElroy returned to the White House on January 1, 1884, to assist in New Year's Day celebrations. McElroy then broke with the tradition of suspending social events during Lent, and she held weekly open-house receptions in the spring during which a hostess would be assigned to each room, culminating in an elaborate luncheon in the family quarters.

Arthur never gave McElroy formal recognition as a first lady out of respect for his late wife. Despite this, she proved to be a popular and competent hostess. Future First Ladies used the social function procedures she and her brother developed for decades. Her responsibilities typically involved hosting more formal events, as Arthur would host informal dinners himself if families were not attending. McElroy was noted for her hospitality while hosting, using a more personal style with guests.