William Leahy was born on May 6, 1875, in Hampton, Iowa. He entered the U.S. Naval Academy on May 19, 1893, and graduated on June 2, 1897. Leahy was serving on the battleship USS Oregon (BB-3) and participated in the Battle of Santiago on July 3, 1898, during the Spanish-American War. He completed his required two years of sea duty and was commissioned an Ensign in the U.S. Navy on July 1, 1899. Leahy served on 11 different ships before serving as an instructor at the U.S. Naval Academy from March 1907 to July 1910, with a few short interruptions to serve on the Navy Rifle Team and to serve a short cruise on the armored cruiser USS California (ACR-6). After completing instructor duties at the Academy, Leahy again served aboard the USS California from July 1910 to October 1912. He was Chief of Staff to the Commander of Naval Forces during the Occupation of Nicaragua from October to December 1912, and then became Assistant Director of Target Practice and Engineering Competitions. Leahy served in the Bureau of Navigation from August 1913 to September 1915, when he took command of the gunboat USS Dolphin (PG-24). He served as Executive Officer of the battleship USS Nevada (BB-36) from April 1917 to April 1918, when he took command of the transport ship USS Princess Matoika (ID-2290), transporting troops to France from April to June 1918 for service in WWI. After the war, Leahy commanded two other ships and served as commander of Mine Squadron One, Director of Officer Personnel in the Bureau of Navigation, Chief of the Bureau of Ordnance, Commander of Destroyers Scouting Force, Chief of the Bureau of Navigation, Commander of Battleships Battle Force, and Commander in Chief of Battle Force before becoming Chief of Naval Operations in January 1937. ADM Leahy retired from the Navy on August 1, 1939. After his retirement, he became Governor of Puerto Rico, serving from September 1939 to November 1940, before serving as Ambassador to Vichy France from 1941 to May 1942. ADM Leahy was recalled to active duty in July 1942, and served as Chief of Staff to the Commander in Chief under President Roosevelt, a position created for him by the President. He was promoted to Fleet Admiral of the Navy on December 15, 1944, and left regular active duty in January 1949, although as a Fleet Admiral, he remained on the active duty rolls of the Navy until his death on July 20, 1959.

His Navy Cross Citation reads:

The Navy Cross is awarded to Captain William D. Leahy, U.S. Navy, for distinguished service in the line of his profession as commanding officer of the U.S.S. Princess Matoika, engaged in the important, exacting and hazardous duty of transporting and escorting troops and supplies through waters infested with enemy submarines and mines.