On December 15, 1928, Hitchcock married Margaret Lederle "Peggy" Mellon (1901–1998), the younger daughter of businessman
William Larimer Mellon, in
New York City. They had four children together – daughters Louise Eustis Hitchcock and Margaret Mellon Hitchcock, and
twin sons Thomas Hitchcock III and William Mellon Hitchcock. For several years, Hitchcock was employed in New York by venture capitalist and socialite George Gordon Moore. He also coached a notable polo team (including
W. Averell Harriman), known as the San Carlos Cardinals, at Moore's Rancho San Carlos (now the
Santa Lucia Preserve) in
Carmel, California. In 1937, with fellow polo player
Robert Lehman, Hitchcock became a partner in the
Lehman Brothers investment firm. Serving as a
lieutenant colonel in the
United States Army Air Forces during
World War II, Hitchcock was assigned as an assistant air attaché to the U.S. Embassy in
London, England. In that capacity, he was instrumental in the development of the
P-51 Mustang fighter plane, particularly in replacing the original Allison engine with the Packard-built Rolls-Royce Merlin. Hitchcock was killed while piloting one such aircraft near
Salisbury in
Wiltshire, when he was unable to pull out of a dive while doing tests. His death was reported to his family by fellow St. Paul's alumnus
John G. Winant, then serving as
United States Ambassador to the United Kingdom. ==Legacy==