William Backhouse Astor Sr. (September 19, 1792 – November 24, 1875) was an American business magnate who inherited most of his father John Jacob Astor's fortune. He worked as a partner in his father's successful export business. His massive investment in Manhattan real estate enabled major donations to the Astor Library in the East Village, which became the New York Public Library.

William was born in New York City on September 19, 1792, and named after William Backhouse, a friend of his father who was a New York merchant.^[1] He was a son of fur-trader John Jacob Astor (1763–1848) and Sarah Cox Todd (1761–1834).^[1] His seven siblings were Magdalena (1788–1832), Sarah (1790–1790), John Jr. (1791–1869), Dorothea (1795–1874), Henry (1797–1799), Eliza (1801–1838), and an unnamed brother who died shortly after his November 13, 1802, birth.^[2]

He attended local public schools. His spare hours and vacations were employed in assisting his father in the store. Official records show that he matriculated with the class of 1811 of Columbia College, but did not graduate from the institution.^[3] At Columbia, he was a member of the Peithologian Society and Philolexian Society, both undergraduate literary societies.^[4]

Instead, at age sixteen, he was sent to the University of Göttingen in Germany, where he joined the German Student Corps Curonia of the Baltic German students; later he moved to the University of Heidelberg. He chose as his tutor a student, afterward known as the Chevalier Christian Charles Josias von Bunsen, with whom he also traveled.^[5]

In 1815, he returned to the United States and entered partnership with his father, who changed the name of his firm to John Jacob Astor & Son and engaged in the China trade.^[5] William's elder brother John Jacob Astor Jr. was sickly and mentally unstable, and had no part in the business.^[6] He worked there until his father's death. One source argued that his role in the company was never anything more than as "an industrious and faithful head clerk", despite his official title of head of the firm's chief subsidiary, the American Fur Company, in its last several years of its ownership by Astor & Son.^[6]

Although William's fortunes grew with his father's company, he became a truly wealthy man when he inherited the estate, worth around $500,000 (equivalent to $15.8 million in 2023), of his childless uncle Henry Astor I, who died in 1833. When John Jacob Astor Sr. died in 1848, William became the richest man in America.^[7][8]

On May 20, 1818, William married Margaret Alida Rebecca Armstrong (1800–1872), the daughter of Senator John Armstrong Jr. and Alida (née Livingston) Armstrong and sister of Horatio Gates Armstrong. Her mother, a member of the prominent Livingston family, was the youngest child of Judge Robert Livingston and Margaret (née Beekman) Livingston as well as the sister of Chancellor Robert R. Livingston and Secretary of State Edward Livingston.^[12] Her father, John Armstrong Jr. was President James Madison's second Secretary of War.^[13] Together, William and Margaret had seven children:^[14]

1. Emily Astor (1819–1841), who married Samuel Cutler "Sam" Ward (1814–1884),^[15] a financier/lobbyist/author, on January 5, 1838, and had two children.^{[lower-alpha 1]}
2. John Jacob Astor III (1822–1890),^[16] who married Charlotte Augusta Matilda Gibbes (1825–1887)^[17] on December 9, 1846, and had one son.^[16]
3. Mary Alida Astor (1823–1881), who married John Carey (1821–1881) on April 16, 1850, and had three children.^[18]
4. Laura Eugenia Astor (1824–1902), who married Franklin Hughes Delano (1813–1893),^[19] on September 17, 1844 (no issue).^{[20][21][lower-alpha 2]}
5. William Backhouse Astor Jr. (1829–1892),^[22] who married socialite Caroline Webster "Lina" Schermerhorn (1830–1908) on September 20, 1853, and had five children.^[23]
6. Henry Astor III (1830–1918),^[24] who married Malvina Dinehart (1844–1918) in 1871 after which he was estranged from his family (no issue).^[25]
7. Sarah Todd Astor (1832–1832), who died in infancy.

As a wedding gift, Astor gave Franklin and Laura Delano the southernmost 100 acres of Rokeby estate.^[26] The estate came to be known as "Steen Valetje" (which means "little stone valley" in Dutch).^[27]

Margaret Astor died on February 15, 1872, William Astor survived his wife by three years dying on November 24, 1875, in his townhouse at Lafayette Place in New York City.^[14] He was buried next to his wife in the Astor vault at Trinity Church Cemetery, designed by Frederick Clarke Withers, in New York City.^[28] In 1866 he conveyed the adjoining 142 acres of Rokeby to his son, Henry. Henry Astor built a brick dwelling on this land, but in 1873 conveyed the property to Laura.

Astor's local newspaper The New York Times eulogised,^[14]

Mr. William B. Astor. an illness of four days ends an honored and successful life the public events in Mr. Astor's career a ripe scholar and philanthropic man. Mr William B. Astor, after an illness of only a few days, died at his residence in this City yesterday at 9:30 A.M., aged eighty three years. Mr. Astor was in his usual good health, except for a slight cold, until Saturday of last week. On that morning his cold began to trouble him and occasioned a severe cough.^[14]

• Rokeby (Barrytown, New York)
• "Steen Valetje"