Onstott began writing "Mandingo" when he was 65 years old. He based some of the events in the novel on "bizarre legends" he heard while growing up: tales of slave breeding and sadistic abuse of slaves. Having collaborated with his adopted son on a book about dog breeding, he decided to write a book that would make him rich. "Utilizing his son's anthropology research on West Africa, he handwrote "Mandingo" and his son served as editor. Denlinger's, a small Virginia publisher, released it and it became a national sensation." He was invited to write an article for "True: The Man's Magazine" in 1959 about the horrors of slavery.