Yamamoto graduated from the Japanese Naval Academy in 1904, and a year later he was wounded in action at the Battle of Tsushima (Tsushima, Battle of) during the Russo-Japanese War. In 1913 he enrolled in the Japanese Naval Staff College, and after graduating in 1916 he was adopted into the Yamamoto family and changed his name. As a lieutenant commander, Yamamoto studied English at Harvard University (1919–21). He then taught at the Japanese Naval Staff College (1921–23) before being sent to Kasumigaura (in Ibaraki prefecture) for flight training in 1924. Promoted to captain, Yamamoto was assigned to another tour in the United States, first as an aide to an admiral and then as a naval attaché in Washington (1926–28). From his time in the United States, Yamamoto took habits and patterns of thought that influenced his later war service. In addition to becoming a relentless poker player, Yamamoto developed a low opinion of American naval officers, considering the U.S. navy a club for golfers and bridge players. On the other hand, he developed a healthy respect for American industrial capacity.

Returning to Japan, Yamamoto embarked on a 10-year period that made him one of Japan's foremost aviation officers. He commanded the aircraft carrier Akagi in 1928. Promoted to rear admiral in 1929, Yamamoto served as chief of the Technological Division of the Naval Air Corps, where he championed the development of fast carrier-borne fighter planes, a program that produced the famous Zero fighters. In 1934 Yamamoto commanded the First Carrier Division, and in 1935 he headed the Japanese delegation to the London Naval Conference, where Japan abandoned 15 years of uneasy naval détente among the world powers. In 1936, as a vice admiral, he became the vice minister of the navy. Yamamoto commanded the First Fleet in 1938, and he became commander in chief of the Combined Fleet in 1939. In these later capacities, Yamamoto used his growing seniority to turn the navy away from battleships, which he viewed as obsolete, in favour of tactics based on aircraft carriers—carrier tactics that he later incorporated into the plan to attack Pearl Harbor.

As the senior seagoing admiral in the Japanese fleet, Yamamoto prepared for war against the United States. Contrary to popular belief, Yamamoto argued for a war with the United States once Japan made the fateful decision to invade the rich lands of Southeast Asia; others in the naval ministry hoped to avoid war with America even while making war with Dutch and British possessions in Asia. When the Japanese emperor Hirohito adopted Yamamoto's view, the admiral focused his energy on the coming fight with the U.S. Pacific Fleet. Well aware of the immense industrial capacity of the United States, but misunderstanding the potential resolve of the American public, Yamamoto asserted Japan's only chance for victory lay in a surprise attack that would cripple the American naval forces in the Pacific and force the United States into a negotiated peace, thereby allowing Japan a free reign in greater East Asia. Any long war with the United States, Yamamoto believed, would spell disaster for Japan. Although he was not the author of the detailed plan to attack Pearl Harbor, he certainly championed it within government circles. On Dec. 7, 1941, his carriers, under the immediate command of Vice Adm. Nagumo Chūichi, scored a stunning tactical victory over the U.S. Pacific Fleet at anchorage in Pearl Harbor. An unbroken string of naval victories followed this attack for six months, and Yamamoto's prestige reached new heights by the late spring of 1942.

Yet the great tactical success of the Pearl Harbor strike obscured a strategic calamity. Far from encouraging the United States to sue for peace, the attack enflamed the American public; the surprise bombing, designed to avert a long conflict with the United States, instead helped ensure a prolonged and total war. Yamamoto stumbled further at the Battle of Midway (Midway, Battle of) (June 4–6, 1942), where he hoped to destroy U.S. ships not caught at Pearl Harbor, notably the U.S. Navy's aircraft carriers. But the strike at Midway failed, partly because the United States had excellent intelligence information regarding Japanese forces but also because Yamamoto's plans were too complex and his objectives confused. The Japanese battle plan included the movement of eight separate task forces, a diversionary attack in the Aleutian Islands, and the occupation of the Midway Islands, all while attempting the destruction of the American carriers. Yamamoto's ensuing campaign for Guadalcanal (Guadalcanal Island) and the Solomon Islands in the South Pacific was not much better, as he refused to commit his forces in anything other than piecemeal fashion as Allied forces there conducted the kind of attrition war Japan could ill afford.

Still, American assessment of Yamamoto was great enough that, when intelligence information revealed the Japanese admiral's flight plan in April 1943, U.S. commanders in the Pacific undertook to ambush and shoot down his plane. On April 18, 1943, during an inspection tour of Japanese bases in the South Pacific, Yamamoto's plane was shot down near Bougainville Island, and the admiral perished.

Yamamoto was Japan's most prominent naval officer during World War II. Despite his relative inexperience at sea in the years before Pearl Harbor, his contribution to naval strategy lies in his early recognition of the effectiveness of carrier-based aircraft in long-range naval attacks. Although he was a better tactician than strategist, he was an unusually gifted and able officer as well as a complex man of sometimes contradictory character.