Leahy, William Daniel (1875–1959), a United States naval officer, diplomat, and statesman. During World War II Leahy was personal chief of staff to two Presidents—Franklin D. Roosevelt, his close friend, and Harry S. Truman—and advised them on military and foreign policies.

Leahy was born in Hampton, Iowa. He graduated from the U.S. Naval Academy in 1897 and served in the Spanish-American War and World War I. He was chief of naval operations, 1937–39. After retiring he held several diplomatic posts, but was recalled to active duty by Roosevelt in 1942. He became an admiral of the fleet in 1944 and retired in 1949.