Maruo graduated from junior high school in March of 1972, but dropped out of senior high school. At the age of 15, he moved to Tokyo and began working for a bookbinder. At 17, he made his first manga submission to Shonen Jump, but it was considered by the editors to be too graphic for the weekly magazine's format and was subsequently rejected. Maruo temporarily removed himself from manga until November of 1980 when he made his official debut as a manga artist in Ribon no Kishi at the age of 24. It was at this stage that the young artist was finally able to pursue his artistic vision without such stringent restrictions over the visual content of his work. Two years later, his first stand-alone anthology, Barairo no Kaibutsu (Rose Colored Monster) was published. Maruo was a frequent contributor to the legendary underground manga magazine Garo.

Like many manga artists, Maruo sometimes makes cameo appearances in his own stories. When photographed, he seldom appears without his trademark sunglasses. Though most prominently known for his work as a manga artist, Maruo has also produced illustrations for concert posters, CD Jackets, magazines, novels, and various other media. Some of his characters have been made into figures as well. Though relatively few of Maruo's manga have been published outside of Japan, his work enjoys a cult following abroad. His book Shōjo Tsubaki (aka Mr. Arashi's Amazing Freak Show) has been adapted into an animated film by Hiroshi Harada with a soundtrack by J.A. Seazer. In Europe. it was marketed under the name Midori, after the main character. It was released on DVD in France by Cinemalta (the DVD includes English subtitles).