Born in Manhattan to architect and landscape designer Charles Platt, William Platt graduated from Harvard College with a B.A. in 1919, going on to earn a Bachelor’s in Architecture from Columbia University in 1923. He served as an officer in the United States Navy Reserve in both World Wars. Platt took over his father’s architectural practice, later partnering with his brother Geoffrey. Together they operated the New York City architectural firm Charles A. Platt, William & Geoffrey until 1933, when they changed the name to William & Geoffrey Platt. The brothers consulted for alterations to Fort Clinton in Stony Point, New York and the old Aquarium in Manhattan’s Battery Park. They collaborated with landscape architect Ellen Shipman, whom their father had mentored, at Longue View House and Gardens, the home of Edgar and Edith Stern in New Orleans. After her death in 1950, William continued as a consultant with the Sterns, adding the Spanish Court, which was lined with fountains and modeled after the Alhambra’s Generalife garden in Spain. In 1958 the brothers collaborated with Thomas Church on the redesign of the Wagner estate in Lakewold, Washington, now Lakewold Gardens. They also designed the Jeannette Kittredge Watson Science and Education Building (1969-1972) at the New York Botanical Garden, major buildings for Deerfield Academy and Smith College, and many private residences across the country.
Platt became a member of the New York Chapter of the American Institute of Architects in 1930, becoming a fellow in 1951. He was also a member of the National Academy of Design in 1948. Platt passed away at the age of 87 at his home in Manhattan.