Coster was born in New York and lived at a large home at 103 East 71st Street. He was one of four children born to Charles Robert Coster (1839–1888)[2][3] and Marie Bay (née James) Coster (1841–1904),[4][5] who were married in 1864.[6] Among his siblings was brother Charles Coster,[a][9] and sister Elizabeth Mary Coster, who married Alfred Egmont Schermerhorn.[b][10] His father was a soldier and public official, who is best known for commanding a brigade at the Battle of Gettysburg.[13]
His maternal grandfather was Augustus J. James of Albany, the brother of theologian Henry James Sr.,[10] making William's mother Marie a first cousin of author Henry James, psychologist William James, and diarist Alice James.[14] His paternal grandparents were John H. Coster and Sarah Adeline (née Boardman) Coster,[15] making his father a first cousin of fellow New York clubman, Harry Coster.[16][c] His great-grandfather, John Gerard Coster,[18] came from Haarlem in the Netherlands to the United States shortly after the Revolutionary War and founded the family fortune through the mercantile firm, "Henry A. & John G. Coster".[19]
Coster became a stockbroker and opened up an office with his brother Charles and John M. Knapp in 1893.[20] He later transferred his seat to Knapp in 1907, and resigned from the firm.[21] He later became a partner in Morgan Drexel.[22] He was known as a speed walker, who could be seen "traversing the sidewalks between the New York Athletic Club and the stock exchange in record time."[20]
In 1908, his brother committed suicide "after he'd been caught bilking his customers out of millions of dollars."[20][21] Although William was accused of wrongdoing, and, along with his other siblings, worked out of the same office as Charles,[7] William himself was eventually declared by the New York Stock Exchange to be completely innocent in the debacle.[20][23]
In 1892, Coster, one of the best-known bachelor clubmen,[4] was included in Ward McAllister's "Four Hundred", purported to be an index of New York's best families, published in The New York Times.[26] Conveniently, 400 was the number of people that could fit into Mrs. Astor's ballroom.[27][22] Coster was a member of the Union Club.[22]
Mary Griswold Coster (1903–1918),[36] who died of pneumonia at age 15.[40]
William Bay Coster Jr. (1908–1945), who rowed on the Pembroke College crew at Oxford and was an air-raid warden in London during World War II.[36]
After living in New York and Paris for many years, Coster died "of a long illness, patiently borne," in Bournemouth, England on December 19, 1918.[1] His widow, who lived in Paris at 5 Rue Vaneau, died at her home, 39 East 79th Street in New York, at the age of 79 in July 1947.[41]
^Charles Coster (d. 1908), was married to Helen Louise Anthon, daughter of Rev. Edward Anthon and Helen (née Post) Anthon.[7][8]
^Elizabeth Mary Coster (1877–1946) was married to Alfred Egmont Schermerhorn (1871–1932), a real estate dealer and member of the prominent Schermerhorn family.[10][11] Their son, Alfred Coster Schermerhorn, was married to romantic fiction writer, Ursula Parrott.[12]
^His grandfather, John H. Coster (better known as a playboy before a businessman), was one of twelve children that married into many prominent families.[17]
^Conklin, George W. (1999). Under the Crescent and Star: The 134th New York Volunteer Infantry in the Civil War, p. 31. Axworthy Publishing. ISBN0-9674985-0-3.