Kojima was born in Yokkaichi, Mie on the same day as Osamu Tezuka. After getting out of junior high school, Kojima painted advertising posters for movie theaters as his source of income.

In 1950, he moved to Tokyo. The post-World War II environment lead to forms of manga meant for impoverished audiences. Kojima created art for kamishibai or "paper play" narrators. Kojima then started to create works for the kashi-bon market.

In 1967, Kojima created Dojinki, his first manga for a magazine. In 1970, he collaborated with writer Kazuo Koike to create Kozure Okami (Lone Wolf and Cub), their most famous work. Koike and Kojima were dubbed "the Golden Duo".

Kojima died on January 5, 2000 at the age of 71.