Born in Wakayama, Japan, he graduated from the Osaka Designers' college in 1979 and a couple of years later joined the studio Annapuru as an animator for the TV series Tomorrow's Joe.
While working there, he saw some animation by Takashi Nakamura in Gold Lightan, an otherwise standard mecha TV series by a rival studio.
Morimoto often collaborated with Nakamura, most notably in Katsuhiro Otomo’s "The Order to Stop Construction" segment of the anthology film Neo-Tokyo.
This opened many doors for him, from working as animation director on Otomo's landmark feature Akira and a chance to direct a short for the Robot Carnival anthology.
Around this time he founded Studio 4°C with producer Eiko Tanaka and fellow animator Yoshiharu Sato.