From an Irish-American family, Leahy was born in Hampton, Iowa, and moved with his parents to Ashland, Wisconsin, as a child. His father Michael Arthur Leahy (1838–1921) was a successful lawyer and Civil War veteran, and William had wanted to attend West Point and follow in his father's footsteps.
However, he attended the United States Naval Academy, graduating 15th in his class of 47 in 1897.
In World War I William Leahy commanded the navy transport Princess Matoika and formed a lasting friendship with Roosevelt, then assistant secretary of the navy. After the war Leahy’s ability brought steady professional advancement: rear admiral in 1930, vice admiral in 1935, and admiral in 1936. His talent for organization was early recognized, and during the post-World War I years he held all three of the navy’s highest administrative offices: chief of the Bureau of Ordnance (1927–1931), chief of the Bureau of Navigation (1933–1935), and chief of Naval Operations (1937–1939).
Leahy’s first career ended with retirement because of his age in August 1939.
His second career began a few months later, when President Roosevelt named him governor of Puerto Rico. He held that post until December 1940, when he was appointed ambassador to the Vichy government that had been established in France after the Germans defeated the French army. There Leahy’s chief task was to stiffen French resistance to German pressure and to keep the French fleet out of German hands. After achieving some success in this difficult assignment, he was recalled in 1942 to fill the newly created position of chief of staff to Roosevelt. As liaison between the president and the armed services, Leahy conferred with Roosevelt daily, presided over the Joint Chiefs of Staff, and participated in almost all the major military and diplomatic decisions of the war years. He was made a fleet admiral in December 1944 and accompanied Roosevelt to the Yalta Conference the following year.
After Roosevelt’s death (April 1945), Leahy was asked by President Harry S. Truman to continue as his personal chief of staff. He retired from that position in 1949, and the following year he published a book of war memoirs, "I Was There" (1950).
While serving on the USS Pensacola, which was based in San Francisco, Leahy met Louise Tennent Harrington, whom he married on 3 February 1904. During Leahy's tenure as Ambassador to France, his wife underwent a hysterectomy which could not be delayed until their pending return to the United States. While recovering in hospital from the operation, Louise Leahy suffered an embolism and died with Leahy at her side on 21 April 1942. After a service at the St Thomas Episcopal Church, she was buried on 3 June 1942 in Arlington National Cemetery.
William and Louise had children, including a son, William Harrington Leahy, who graduated from the U.S. Naval Academy in 1927. He engaged in pre-World War II naval intelligence.
