Etō was born in the Shinjuku district of Tokyo; his father was a banker, and his grandfather (originally from Saga in Kyūshū) was an admiral in the Imperial Japanese Navy.

In the immediate postwar era, he went to high school in Fujisawa, Kanagawa prefecture, where he developed a friendship with future Tokyo governor Shintaro Ishihara, who was one year ahead of him.

He followed this with Dorei no shisō wo haisu (1958) and Sakka ha kōdō suru (1959), in which he argued that a writer's style was directly related to his personal behavior and background.

[1] In 1958, Etō joined a group of young, left-wing writers, artists and composers to form the "Young Japan Society" (Wakai Nihon no Kai) for the purpose of protesting a draconian Police Duties Bill introduced by conservative prime minister Nobusuke Kishi that would have authorized police use of warrantless searches and seizures against left-wing activists.

Etō was a very prolific author, and his books and essays ranged from literary criticism and to postwar political commentary; through taking controversial viewpoints, he also established himself as one of foremost public intellectuals in the print and television.

[6] In 1970, Etō completed Umi ga Yomigaeru, a work on the Russo-Japanese War, which (in August 1977) was made into the first three-hour historical drama to be aired on Japanese television.