Yamaguchi started his career in 1963 as an assistant director at Toei Douga's Tokyo Studio and later transferred to the Production Department where he conducted planning on future series. He planned titles such as Wolf Boy Ken, Ge Ge Ge no Kitarou, Getter Robo, King Kong, and became an anime picture producer, producing titles including Candy, Candy, Story of Maple Town and Tatsu no ko Tarou. As chief of the production section, he produced Sailor Moon.
