William B. Coster

William Bay Coster (1867 – December 19, 1918) was an American banker active in New York during the Gilded Age.

**Early Life**

Coster was born in New York City in 1867 and resided at 103 East 71st Street. He was one of four children of Charles Robert Coster (1839–1888) and Marie Bay (née James) Coster (1841–1904), who married in 1864. His sibling included Charles Coster and Elizabeth Mary Coster, who married Alfred Egmont Schermerhorn.

His father, Charles Robert Coster, was a soldier and public official, known for commanding a brigade at the Battle of Gettysburg. His maternal grandfather, Augustus J. James of Albany, was a brother of theologian Henry James Sr., making William's mother Marie a first cousin of notable figures Henry James, William James, and Alice James.

Through his paternal lineage, Coster descended from John Gerard Coster, who emigrated from Haarlem in the Netherlands shortly after the American Revolutionary War, and established the mercantile firm Henry A. & John G. Coster, which contributed to the family’s wealth.

**Career**

William Coster pursued a career in stockbroking. In 1893, he co-founded a stock exchange office with his brother Charles and John M. Knapp. In 1907, Coster transferred his seat to Knapp and subsequently resigned from the firm. Later, he became a partner at Morgan Drexel. Coster was also recognized for his speed-walking ability, often seen traversing sidewalks between the New York Athletic Club and the stock exchange.

In 1908, his brother Charles committed suicide after financial misconduct involving the bilking of customers. Despite allegations involving William and other siblings, the New York Stock Exchange eventually declared William entirely innocent of wrongdoing related to the incident.

Coster served in the New York National Guard as aide de camp to the First Brigade, attaining the rank of captain in 1896.

**Society Life**

In 1892, William Coster was listed in Ward McAllister's "Four Hundred," an enumeration of New York’s prominent families, published in The New York Times. The "Four Hundred" represented the capacity of Mrs. Astor’s ballroom, indicating social prominence. Coster was a member of the Union Club and was well-known as a bachelor clubman.

**Personal Life**

On October 1, 1900, Coster married Maria "Minnie" Griswold Gray at St. Saviour's Episcopal Church in Bar Harbor, Maine. Minnie was the daughter of Henry Winthrop Gray and Mary (née Travers) Gray, granddaughter of William R. Travers, and a close friend of etiquette author Emily Post. Her parents divorced, and her father remarried in 1889 to Matilda Frelinghuyhsen, daughter of U.S. Secretary of State F. T. Frelinghuysen.

William and Minnie resided initially in New York and later in Paris. They had three children:

- Matilda Gray Coster (1901–1962), who married Stanley Yates Mortimer Jr. in 1925, a grandson of Valentine Hall Jr. and a first cousin of Eleanor Roosevelt. They divorced in 1928, and she subsequently married Luis Martínez de las Rivas in 1932. They later lived in Cuernavaca, Mexico.

- Mary Griswold Coster (1903–1918), who died of pneumonia at age 15.

- William Bay Coster Jr. (1908–1945), who participated in rowing at Oxford and served as an air-raid warden during World War II in London.

William Coster died from a long illness in Bournemouth, England, on December 19, 1918. At the time, he was residing in Bournemouth. His widow, Minnie, lived at 5 Rue Vaneau in Paris and died at 79 in New York City in July 1947.

**Descendants**

Through his daughter Matilda, William Coster was the grandfather of Mathilda Coster Mortimer (1925–1997), who became the Duchess of Argyll upon her marriage to Ian Campbell, 11th Duke of Argyll, in 1963. Mathilda was previously married to Clemens Heller, founder of the Salzburg Global Seminar, and had one child, Lady Elspeth Campbell, who was born in 1967 and lived only five days. The Duke and Duchess remained married until his death in 1973.