Yokoyama came out with the first book for his manga artist debut, which caught Osamu Tezuka's attention. In 1955, Yokoyama had a title serialized in the magazine '''' for the first time, "". In 1956,
Tetsujin 28-go appeared serially in the
shōnen magazine after he resigned from the movie company.
Tetsujin 28-go became a popular work equal to Tezuka's
Astro Boy and its animated adaptation also made a smashing success. This prompted Yokoyama to become a full-time manga artist and to move to
Tokyo the same year. In 1964, he established "Hikari Production", an incorporated company. Making good use of his vast exposure to movies during his previous job, he produced consecutive popular hits in various genres, both in comics and
anime, such as
Iga no Kagemaru,
Akakage,
Sally the Witch,
Giant Robo,
Babel II and so on. With the writing of (1967–1971) and
Sangokushi (1971–1987), he began a new chapter in his career as he drew mostly comics based on original stories with material from China's and Japan's histories. In 1991,
Sangokushi won the prize for excellence from the Japan Cartoonist Association and an animated version was broadcast on TV Tokyo. In July 1997, Yokoyama was hospitalized with
myocardial infarction and had an operation. He returned to work in March the next year. In 2004, while under medical treatment, Yokoyama won the
MEXT Prize of the Japan Cartoonist Association. On the morning of April 15, 2004, Yokoyama suffered burns all over his body due to a fire breaking out in his house. His condition deteriorated and he fell in a
coma. Yokoyama died in the hospital near his home at 10:00 P.M. on the same day, aged 69. ==Style==