He was influenced by friends of his father such as Hasegawa Nyozekan, a circle of people identified with the liberal current of political thought during the period of Taishō democracy.

The person who originally recommended this path to him was his mentor, Professor Shigeru Nambara, who was highly critical of military and bureaucratic obstructions to the growth of a constitutionally defined "national community.

Maruyama first attracted attention from the scholarly community immediately following the war with his famous essay on wartime Japanese fascism, "The Logic and Psychology of Ultranationalism," first published in the widely-read journal Sekai in 1946.

Maruyama continued to write about wartime and contemporary Japanese politics in the late 1940s and early 1950s, until he was forced to take a break from scholarly activities due to his being in and out of hospitals with illnesses in the mid-1950s.

[5] It was not until the late 1950s that Maruyama's earlier essays were anthologized and republished for the first time, bringing him fame and acclaim from a much broader cross-section of the Japanese general public.

[5] Shortly after the shocking ramming of the Treaty through the Diet by Prime Minister Nobusuke Kishi on 19 May 1960, Maruyama emerged as one of the main faces of the anti-Treaty movement.

He also argued that due to Kishi's outrageous actions, ordinary Japanese people needed to support the anti-Treaty protests in order to protect democracy, even if they did not mind the Treaty itself.

[7] Maruyama was most strongly attacked by fellow leftist intellectual Yoshimoto Takaaki, who had a large following among New Left student radicals.