[2] He graduated from the 29th class Imperial Japanese Naval Academy in 1901, ranked 68 of 125 cadets (Japan Center for Asian Historical Records, n.d.).

He served in administrative positions until near the end of the Russo-Japanese War of 1904–1905, when he went to sea again on the destroyer Inazuma and the cruiser Iwate.

While overseas, he was promoted to commander; after the collapse of the Russian Empire, he was recalled to Japan and later became executive officer on the battleship Asahi.

Following the success of this mission, he was promoted to vice-admiral in December 1930 and placed in command of the Chinkai Guard District, in Korea.

Yonai was visiting his mistress in Shinbashi the night the attempted coup d'état began, only a couple of blocks away, but knew nothing of the situation until he returned to base the following morning.

As Navy Minister, Yonai was alarmed by the growing tension between Japan and Great Britain and the United States, at a time when the bulk of the Imperial Japanese Army was tied down in an apparently unending quagmire in China.

His efforts to promote peace made him unpopular with ultranationalist extremists, and (as with Admiral Isoroku Yamamoto) he was the target of several assassination attempts.

However, Yonai supported the construction of the Yamato-class battleships in an effort to maintain a military balance with the world's other two naval superpowers.

Yonai was appointed the Prime Minister of Japan and formed his cabinet from January 6, 1940, largely with the backing of Emperor Hirohito.

Following the German occupation of the Low Countries and France in May–June 1940, the Imperial Japanese Army began to show dissatisfaction with Yonai's anti-German and anti-Italian policy.

In the last few weeks before Japan's surrender, he sided with Prime Minister Suzuki and Foreign Minister Shigenori Tōgō in support of acceptance of the Potsdam Declaration and surrender of Japan in opposition to Army Minister Korechika Anami, Chief of Naval General Staff Admiral Soemu Toyoda and Chief of the Army General Staff General Yoshijirō Umezu.

According to his interpreter Suichi Mizota, in March 1946 Bonner Fellers asked him to make Tōjō bear all responsibility for the Greater East Asia War.

Yonai suffered from high blood pressure most of his life, but died of pneumonia on 20 April 1948 at the age of 68.

Hence, on August 8, 1939, at the five-ministry commission that was intended to make a plan for war, the Minister of Finance, Ishiwata, asked Yonai, "Is it possible for the Imperial Japanese Navy to triumph over America and Britain?"

After the historical triumph of the Battle of the Japan Sea in 1905 during the Russo-Japanese War, the Imperial Japanese Navy was the world's third strongest.

On February 26, 1936, there was an attempted coup d'état led by young officers of the Imperial Japanese Army.

Because of this immediate announcement, navy officers in the Sasebo Naval District were compelled to stop participating with the rebellion troops.

This reunion was named Ichi-Roku Kai, which means sixteenth gathering, because Yonai's appointment and resignation as a Prime Minister both happened on the 16th day of their months.

"[11] This action is extremely rare because having presented the belongings of the Emperor is the supreme honor and the utmost expression of amiability.