- Profession: Entrepreneur.
- Residences: New York.
- Relation to Mahler:
- Correspondence with Mahler: No.
- Born: 12-07-1829, Manhattan, New York, America.
- Died: 25-04-1892, Paris, France.
- Buried: Trinity Church cemetery, New York, Manhattan, America.
William Backhouse Astor, Jr. was a businessman, racehorse breeder/owner, and yachtsman. He was also a member of the prominent Astor family. While his elder brother financier/philanthropist John Jacob Astor III (1822–1890) was head of the English line of Astors, William Jr. was the patriarch of the male line of American Astors.
A well-liked man, Astor graduated from Columbia College in 1849. In 1853 he married the socially ambitious Caroline Lina Webster Schermerhorn Astor (1830-1908), who reigned over New York and Newport society as simply "the Mrs. Astor." Her parents were Abraham Schermerhorn and Helen White.
- Emily Astor (1854–1881), who married sportsman/politician James John Van Alen and had three children.
- Helen Schermerhorn Astor (1855–1893), who married diplomat James Roosevelt "Rosey" Roosevelt (half-brother of President Franklin Delano Roosevelt) and had two children.
- Charlotte Augusta Astor (1858–1920), who married James Coleman Drayton and had four children, later married George Ogilvy Haig.
- Caroline Schermerhorn "Carrie" Astor (1861–1948), who married Marshall Orme Wilson (brother of banker Richard Thornton Wilson, Jr. and socialite Grace Graham Wilson) and had two sons.
- John Jacob "Jack" Astor IV (1864–1912), who married socialite Ava Lowle Willing and had two children, later married socialite Madeleine Talmage Force (sister of real estate businesswoman/socialite Katherine Emmons Force) and had one son.
William Jr. had little interest in society parties, and Lina would try to keep him at his club late to prevent him coming home and sending the orchestra out and his children to bed. William Backhouse Astor, Jr. died of an aneurysm in Paris, France. He was buried in Trinity Church Cemetery in Washington Heights, New York.