He is best known for his violent, artful seinen manga, notably Lone Wolf and Cub (with Goseki Kojima, 1970–6), Lady Snowblood (with Kazuo Kamimura, 1972–3) and Crying Freeman (with Ryoichi Ikegami, 1986–8), which – along with their numerous media adaptations − have been credited for their influence on the international growth of Japanese popular culture.

Early in Koike's career, he studied under Takao Saito (the creator of Golgo 13), and served as a writer on the series.

Another series written by Koike, Crying Freeman, which was illustrated by Ryoichi Ikegami, was adapted into a 1995 live-action film by French director Christophe Gans.

[2] In 2011, Koike announced his intention to write a magical girl manga series titled Maho Shojo Mimitsuki Mimi no QED.

[3] Koike started the Gekiga Sonjuku, a college course meant to teach people how to be a manga artist.