The younger son of Congressman Charles S. Dewey and his wife, Marie Suzette de Marigny Hall Dewey, and also a distant relative of New York Governor Thomas E. Dewey, Dewey was born in Chicago. He was educated in Switzerland at Institut Le Rosey, before attending St. Paul's School (Concord, New Hampshire). He graduated from Yale University where he studied French history and was a member of the Berzelius Secret Society. Later, Dewey also attended the University of Virginia School of Law. He was shot to death in a case of mistaken identity by Viet Minh troops in September 1945 when they believed him to be French. He was the first American fatality in French Indochina following WWII. He has a cenotaph located in Arlington National Cemetery, Arlington, Arlington County, Virginia.