While in Ibiza, Cormac completed revisions of his next novel, Outer Dark. In 1967, he returned to the United States and moved to Tennessee. Outer Dark was published by Random House in 1968.

Outer Dark is a starkly evocative novel set in an unspecified place in Appalachia, sometime around the turn of the century. A woman bears her brother's child, a boy. He leaves the baby in the woods and tells her he died of natural causes. Discovering her brother's lie, she sets forth alone to find her son. Both brother and sister wander separately through a countryside being scourged by three terrifying and elusive strangers, headlong toward an eerie, apocalyptic resolution.