Having his first hit in the late 60's/early 70's with Otoko Ippiki Gaki-Taishō, which was one of the first two major hits in the Weekly Shonen Jump magazine (alongside Harenchi Gakuen). It was the first series from the magazine to publish twenty collected volumes (at a time when a lot of the material in the magazine was not immediately collected or even collected at all). He would continue publishing numerous serials in the magazine (though none that ran as long) through the 70's and 80's. Altogether his works in that magazine alone were collected in over sixty volumes.
From this point on, he moved more directly into the seinen market, though he had already found early success there in the 70's. Publishing well-received works in a number of Shueisha magazines, including Monthly Shonen Jump and Weekly Playboy, his series Ore no Sora: Keiji Hen received top-billing on the very first issue of Weekly Young Jump. Though the magazine's mascot took the cover itself as he often did in those early years, Motomiya's series is clearly displayed by itself in large font just under the magazine title.
Becoming yet another major magazine where he was one of the first stars, he published a number of works in this magazine over four decades including one of his most notable successes, Salaryman Kintarō. Beginning publication in the early 90's, it became one of the best-selling manga in the seinen market and including various sequels, has been collected in over forty-five volumes.
Over forty years after his first major success, proving himself still relevant in 2011, Motomiya was once again given the honor of ushering in a new magazine when he was given the cover of the debut issue of Grand Jump where he has continued publishing works in the current decade (alongside Weekly Young Jump).
A number of Motomiya's assistants would go on to be major creators in manga themselves (often in the very same magazines as Motomiya himself):
- Yoshihiro Takahashi (began working as an assistant in 1971, while Hiroshi was working at Weekly Shonen Jump, it wasn't until 1976 Yoshihiro would get his first serial in the magazine but he would end up publishing over 90 tankobon volumes worth of material for various Jump magazines)
- Tatsuo Kanai (working as an assistant of Hiroshi's for about two years, his first Weekly Shonen Jump serial began in 1977, he primarily remained an artist rather than a writer and published a number of Jump Comics series, though his first remained his best known)
- Masami Kurumada (Motomiya's Otoko Ippiki Gaki-Taishō played a role in attracting Kurumada to manga, though he would primarily work as an assistant of Ko Inoue in Jump, he also worked under Motomiya before debuting with his first Jump series in 1974 and subsequently making two of the most successful franchises in the magazine among others)
- Tetsuya Saruwatari (worked as an assistant for both Shinji Hiramatsu and Hiroshi Motomiya around the period of 1979-1981, he debuted his first series Weekly Shonen Jump in 1982 but never made it in that magazine, but has since published well over 100 tankobon's worth of material in Shueisha's seinen manga magazines, for decades often appearing alongside Motomiya in magazines including Weekly Young Jump, Grand Jump and Business Jump)
- Tatsuya Egawa (worked for Motomiya for four months, he went on to have a number of long-running series for different magazines including Weekly Shonen Jump, Super Jump and Big Comic Spirits)