Born in Pasadena, Saul grew up in Whittier, California, and graduated from Whittier High School in 1959. He went on to several colleges, variously majoring in anthropology, liberal arts and theater, but never obtained a degree. After leaving college, Saul decided to become a writer, and spent fifteen years working in various jobs while trying to improve his craft.

In 1976, Dell Publishing contacted him and asked if he'd be interested in writing a psychological thriller. The resulting novel, Suffer the Children, appeared on all the best-seller lists in the United States and even reached the number one spot in Canada. In addition to his novels, Saul has had several one-act plays produced in both Los Angeles and Seattle.

Saul lives part-time in the Pacific Northwest, both in Seattle and in the San Juan Islands, and has a residence on the Big Island of Hawaii. Saul is openly gay. He lives with his partner of 32 years, who has collaborated on several of his novels. He is a frequent speaker at the Maui Writers' Conference.