When he was offered a scholarship to the University of California, he declined it, believing he could educate himself more effectively on his own. At the same time, he worked at various jobs, mostly manual labor, including in a lumber mill, on a plantation, in construction, as a quarry worker, and as a window cleaner.

He began publishing his first short stories and poems at the age of seventeen. The fantastic worlds for which he became renowned unfold throughout his short stories and novellas. Their imagery is drawn from the realm of imagination—the dreamlike domain of what is now called fantasy—which he enriched with elements of horror, the bizarre, and science fiction.

Smith employed an elaborate, highly ornate style, embellished with archaic vocabulary, to create a dark, baroque mythology of decadence, decay, and death, in which magic plays the central role. He was a passionate advocate of fantasy and horror literature in all its artistic forms, including poetry, fiction, painting, and sculpture.

His correspondence with H.P. Lovecraft proved decisive for his development. Lovecraft himself regarded Smith as the finest writer of horror fiction of his time. Although strongly influenced by Lovecraft, Smith created mythologies entirely his own, populating original fictional worlds such as the legendary realms of Averoigne, Atlantis, Hyperborea, and the continent of Zothique. He also expanded the Cthulhu Mythos by introducing the Book of Eibon, the dark deity Tsathoggua, and other mythical beings that Lovecraft himself would later incorporate into his own works.

Clark Ashton Smith died in 1961.