Of farming origin in Choshu domain, he studied under Yoshida Shoin at the Shoka Sonjuku. In 1863, along with Inoue Kaoru and others, he secretly traveled to Britain and experienced firsthand the superiority of Western civilization, shifting his thinking from expulsion of foreigners to opening Japan. After the Boshin War he joined the Meiji government, participated in the Iwakura Mission, and observed Western political institutions. In 1882 he traveled to Europe alone to conduct constitutional research in Germany and Austria, playing a central role in drafting the Meiji Constitution (1889), modeled on Prussian constitutional monarchy. With the establishment of the cabinet system in 1885 he became the first Prime Minister of Japan. He organized four cabinets and built the political foundations of Meiji Japan. He also played active roles in diplomacy, including the Treaty of Shimonoseki (1895) after the Sino-Japanese War and the Anglo-Japanese Alliance (1902). Ito is rated as the politician with the most experience as Prime Minister in the Meiji government and the person who completed the institutional framework of the modern Japanese state. On October 26, 1909, he was assassinated at Harbin Station in Korea by Korean independence activist An Jung-geun. He was sixty-eight. He also served as first president of the Privy Council and first Resident-General of Korea.

Practical and realistic, with an exceptionally fine sense of political balance. His ability to accomplish the great task of drafting a constitution—the very foundation of the state—was outstanding. In his private life he had a bold side, and he was a man of complex character.

The enactment of the Meiji Constitution and the establishment of the cabinet system are Ito's greatest achievements and became the foundations of modern Japan. The political institutions he established as first Prime Minister are the prototype of Japan's modern governance system. His portrait appeared on the 1,000-yen banknote for a period, and he is known to the world as a figure symbolizing modern Japanese history.