He graduated from Yale College with a Bachelor in 1900, and an Master of Arts in 1904.
He became president of the board of trustees of the Metropolitan Museum of Art, and founded the Hearth and Home Corporation to provide housing in downtown Manhattan, New York City. They had a son, William Sloane Coffin, Junior., who became a clergyman and a noted peace activist. Coffin, Senior was a director of the family"s furniture and rug business, West. Sloane Company, and later its vice-president
He was elected a trustee of the Metropolitan Museum of Art in 1924, and in 1931 became president of the board of trustees.
West. Sloane acquired the California Furniture Company, and in 1925 Coffin created a subsidiary, the Company of Master Craftsmen, to make Colonial Revival furniture in a factory in Flushing, Queens. Sloane was heavily involved in manufacturing and selling this style, and had another subsidiary, the Oneidacraft Company in Oneida, New York, which made it as well.
Coffin was involved in purchasing real estate properties from Trinity Church and redeveloping them. These houses are now located in the Charlton-King-Vandam Historic District.
He founded the Hearth and Home Corporation, of which he was president, in order to renovate older buildings near downtown Manhattan to provide housing for middle-class New Yorkers, which Coffin saw as a solution to the "apartment house problem" of the late 1910s and early 1920s.
In 1920 Hearth and Home purchased the entire block of mid-19th century row houses bounded by MacDougal, Sullivan, West Houston and Bleecker Streets and renovated those on MacDougal and Sullivan into spacious apartments, with the backyards of the buildings connected to form a common garden. The New York Times said about the project on January 30, 1921 that " development made a real contribution to the solution of the housing problem and is an excellent example of what can be done to other properties in the city, and the rehabilitation of homey old buildings." The buildings were sold to individual owners in 1924, but with covenants guaranteeing aesthetic continuity and no future redevelopment of the site. These houses now make up the MacDougal-Sullivan Gardens Historic District.
Coffin died on December 16, 1933 at his home in Manhattan, New York City from a coronary thrombosis.
He was buried in Sleepy Hollow Cemetery.