[9][10] Yuichiro Hayashi, Munehisa Sakai who became MAPPA's main directors, and Sayo Yamamoto, his direct protégé at Madhouse, are also people whose talents were discovered by Maruyama.

[16] It was not a job he particularly wanted, but he was fascinated by animation after experiencing culture shock by Osamu Tezuka, the head of Mushi Pro.

[6][16] In 1972, Maruyama left Mushi Productions, which was facing a financial crisis, to found Madhouse, along with his colleagues including Osamu Dezaki, Rintaro, and Yoshiaki Kawajiri, and in 1980, he became the president.

[1][6][17] Since then, he has focused on planning, producing and production design for numerous Madhouse TV series, OVAs and theatrical film.

[11] The 1993 film Jūbē Ninpūchō, produced by him and directed by Yoshiaki Kawajiri, was released on video in North America under the title Ninja Scroll, and sold nearly half a million copies.

Maruyama has also planned and produced some of the best-known and rising films of famous directors, such as all of Satoshi Kon's films (Perfect Blue, Millennium Actress, Tokyo Godfathers, Paprika), Mamoru Hosoda's The Girl Who Leapt Through Time and Summer Wars, and Sunao Katabuchi's Mai Mai Miracle.

[22] From 2010 to 2011, Iron Man, Wolverine, and Blade, three works from the Marvel Anime Project, of which Maruyama is one of the planners, were broadcast on Animax in Japan.

[3][17] Then in April 2016, Maruyama handed over the presidency of MAPPA to founding member Manabu Otsuka and newly established Studio M2, specializing in planning and pre-production.