Shea was born to Irish parents in Los Angeles in 1946. There he frequented Venice Beach and the Baldwin Hills for their wildlife. He attended UCLA and Berkeley
He hitch-hiked twice across the US and Canada. At a hotel in Juneau, he chanced on a battered book from the lobby shelves: Cugel the Clever, by Jack Vance.
Four years later, after a brief first marriage and a year spent hitch-hiking through France and Spain, he wrote an hommage to Jack Vance, for which Vance graciously declined to share the advance offered by DAW Books. Shea's first novel, A Quest for Simbilis (1974), was an authorized sequel to Vance's The Eyes of the Overworld.
Subsequently Shea ranged all over the L.A. Basin, painting houses and teaching Adult ESL by night. In 1978 he met his second wife, artist and author Lynn Cesar. They had two children, Adele and Jacob.
Shea moved to the Bay Area where (prior to 1987) he held a variety of occupations, including instructor of languages, construction laborer, and night clerk in a Mission District flophouse.
Shea was quiet for a few years but re-emerged with the collection of four linked novellas Nifft the Lean (1982), where he showed he had developed the exotic style of Vance (perhaps influenced by Clark Ashton Smith) plus the ingenuity of Fritz Leiber's Gray Mouser stories to produce an extravagant quest novel. It received a World Fantasy Award).
Shea followed up with The Color out of Time (1984), (an homage to H. P. Lovecraft's "The Colour out of Space" in which Shea only borrows the setting background, not attempting to pastiche Lovecraft's style); and In Yana, the Touch of Undying (1985), about a vain opportunist's search for immortality in a land of fable.
Polyphemus (1987) is a collection of deft science fiction and horror stories published by Arkham House, in which many of the stories were multiple Hugo Award and Nebula Award finalists. Some betray the possible stylistic influence of Stephen King.
The Mines of Behemoth (1997), a fifth Nifft story originally serialised in Algis Budrys' Tomorrow Speculative Fiction magazine, continues the adventures of Nifft, as does the novel The A'rak (2000). The Nifft stories, examples of the "sword-and-sorcery" genre modeled on Jack Vance, are notable for their imaginative depiction of the world of demons, and their blend of horror, flowery diction, and occasionally crude humor.
Shea's work overlaps the science fiction and fantasy genres, e.g., demons and alien that act as endoparasites.
Shea's interest in Lovecraft's Cthulhu Mythos has continued throughout his career. Copping Squid and Other Mythos Tales (2010) is a collection of such tales.