Emperor Meiji granted Prince Naruhiko the title Higashikuni no miya and permission to start a new branch of the imperial family on November 3, 1906. Prince Higashikuni Naruhiko married the ninth daughter of Emperor Meiji, Princess Toshiko (May 11, 1896 - March 5, 1978), on May 18, 1915. The couple had four sons.

Prince Higashikuni Naruhiko was a career officer in the Imperial Japanese Army. In 1908, he graduated from the Imperial Japanese Army Academy and, in 1914, he graduated from the Army War College. He was commissioned a captain in the 29th Infantry Brigade, and promoted to major in the IJA 7th Division in 1915.

Prince Higashikuni then studied military tactics at the École Spéciale Militaire de Saint - Cyr in Paris France, from 1920 to 1926. Always somewhat of a rebel, Prince Higashikuni's behavior in Paris scandalized the Imperial Court. He left his wife and children in Japan, and the death of his second son did not prompt his return. He had a French mistress, enjoyed fast cars and high living. In 1926, the Imperial Household Ministry dispatched a chamberlain to Paris to collect him.

Upon his return to Japan, he was assigned to the Imperial Japanese Army General Staff Headquarters and eventually rose to the rank of major general, having successively served as commander of the 5th Infantry Brigade (1930 – 1934), the IJA 4th Division (1934 – 1937), and after the start of the Second Sino - Japanese War, the Imperial Japanese Army Air Service (1937 – 1938), and the IJA 2nd Army stationed in China (1938 – 1939).

According to a memo discovered by historian Yoshiaki Yoshimi, Higashikuni also authorized the use of poison gas against Chinese on 16 August 1938.

Promoted to full general, The prince was awarded the Order of the Golden Kite, 1st Class in 1940.

Before the start of the Second World War, on 15 October 1941, outgoing Prime Minister Fumimaro Konoe proposed Prince Higashikuni to Emperor Shōwa as his successor for prime minister. Konoe believed that only a member of the Imperial Family with a distinguished military background could restrain the pro-war faction led by Generals Hajime Sugiyama, Hideki Tōjō, and Akira Mutō). Prince Higashikuni was also the choice of both Chief of staffs of the Army and the Navy.

However, both Emperor Shōwa and the Lord Privy Seal, Kido Koichi, believed that it would be inappropriate for a member of the Imperial Family to serve as he could be blamed for anything which went wrong in the war. Thus, two days later, Hirohito chose General Hideki Tōjō as Prime Minister despite the wish of the Navy and the Army, who would have preferred Prince Higashikuni. In 1946, he explained this decision : "I actually thought Prince Higashikuni suitable as chief of staff of the Army; but I think the appointment of a member of the imperial house to a political office must be considered very carefully. Above all, in time of peace this is fine, but when there is a fear that there may even be a war, then more importantly, considering the welfare of the imperial house, I wonder about the wisdom of a member of the imperial family serving [as prime minister].".

Six weeks later, Japan attacked Pearl Harbor. During the early stages of the Pacific War, Prince Higashikuni served as commander of the Home Defense Command from 1941 to 1944. Files of the Investigative Records Repository (IRR) made public in 2002 show that it was in fact he who ordered the execution of the Doolittle Airmen.

Prince Higashikuni remained steadfast in his opposition to the war with the western powers, and was part of the conspiracy (with Prince Asaka, Prince Takamatsu, and former Prime Minister Konoe) which finally ousted Tōjō in July 1944 following the fall of Saipan to American forces. The American researchers with SCAP also found out that he had planned towards the end of the war to depose Hirohito, placing the minor Akihito on the throne instead, governing the country with himself as regent.

After the course of the war turned against Japan, and the decision was made to accept the Potsdam Declaration, Emperor Shōwa appointed Prince Higashikuni to the position of Prime Minister of Japan on August 16, 1945, replacing Admiral Kantarō Suzuki. The mission of the Higashikuni cabinet was twofold: first, to ensure the orderly cessation of hostilities and demobilization of the Japanese armed forces; and second, to reassure the Japanese people that the imperial institution remained secure. Prince Higashikuni resigned in October over a dispute with the American occupation forces over the repeal of the 1925 Peace Preservation Law.

On February 27 and March 4, 1946, Prince Higashikuni gave interviews to theYomiuri - Hochi and New York Times newspapers in which he claimed that many members of the imperial family had approved Emperor Hirohito’s abdication, with Prince Takamatsu serving as regent until Crown Prince Akihito came of age. In the government, only Prime Minister Kijuro Shidehara and the Imperial Household Minister opposed this.

In 1946, Prince Higashikuni asked Emperor Shōwa for permission to renounce his membership in the Imperial Family and become a commoner. Emperor Shōwa denied the request. However, along with other members of the imperial branch families (shinnōke and ōke), Prince Higashikuni lost his title and most of his wealth as a result of the American occupation’s abolition of the princely houses on October 17, 1947.

As a private citizen, Higashikuni operated several unsuccessful retail enterprises (including a provisions store, second - hand goods store, and dressmaker's shop). He even created his own new Zen Buddhism based religious sect, the Higashikuni - kyo, which was subsequently banned by the American occupation authorities.

The former prince became the honorary chairman of the International Martial Arts Federation (IMAF) in 1957, and honorary president of several other organizations.

In 1958, Higashikuni published his wartime journals under the title, Ichi Kozoku no Senso Nikki (or The War Diary of a Member of the Imperial Family). He published his autobiographical memoirs, Higashikuni Nikki, in 1968.

Former Prince Higashikuni Naruhiko died of heart failure in Tokyo on January 20, 1990 at the age of 102, having outlived his wife, two of his sons, his siblings, and his nephew, Emperor Shōwa. Higashikuni is today mainly remembered as Japan's first postwar prime minister. He was one of the longest lived prime ministers of all time, along with Willem Drees, Christopher Hornsrud and Antoine Pinay.