Initially working as a sign painter, Seo began dabbling in drawing animation by working at a toy film company that made short movies for home entertainment. Although his most famous films were propaganda for Japan during World War II, Seo's political sympathies were leftist, and early on, he was actually a member of the Proletarian Film League of Japan, where he helped out on such animated films as Sankichi no Kūchū Ryokō. In 1931, he was arrested for his activities, tortured, and spent 21 days in jail. . . . After the war, Seo joined Nihon Manga Eigasha and made the film Ōsama no Shippo as a pro-democracy anime in 1949, but when Tōhō, which was supposed to distribute it, found it politically too leftist, the film was left without a distributor. Nihon Manga Eigasha went bankrupt, and Seo, finding the conditions for animation in the immediate postwar too difficult, left the industry and became an illustrator for children's books.