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.

Dying Earth

• A Quest for Simbilis (1974)

Nifft

• Nifft the Lean (1982). There is also a limited edition (440 copies) reprint by Darkside Press (1994)
• The Mines of Behemoth (1997)The Incompleat Nifft (omnibus, 2000)
• The A'rak (2000)

Other novels

• The Color Out of Time (1984)
• In Yana, the Touch of Undying (1985)
• I, Said the Fly (1993). A limited edition of 300 copies from Silver Salamander Press.
• The Extra (2010). Based on Shea's original short story of the same title, this novel is intended as the first of a trilogy.

Novels by Shea as-yet-unsold include Cannyharme, Momma Durtt and The Plunderers.

Collections

• Polyphemus (1987)
• The Autopsy and Other Tales (2008) [This short story collection from Centipede Press also includes the complete Lovecraftian novel The Color Out of Time].
• Copping Squid and Other Mythos Tales (edited by S.T. Joshi) (Perilous Press, 2010)

Chapterbooks

• Fat Face (1987)

Short stories

• "The Angel of Death" (1979)
• "The Autopsy" (1980)
• "Polyphemus" (1981)
• "Nemo Me Impune Lacessit" (1982)
• "That Frog" (1982)
• "The Horror on the #33" (1982)
• "The Fishing of the Demon-Sea" (1982)
• "Come Then, Mortal, We Will Seek Her Soul" (1982)
• "The Goddess in Glass" (1982)
• "The Pearls of the Vampire Queen" (1982)
• "Shag Margold's Eulogy of Nifft the Lean, His Dear Friend" (1982)
• "Grunt-12 Test Drive" (1983)
• "Creative Coverage, Inc." (1983)
• "Uncle Tuggs" (1986)
• "Fill It With Regular" (1986)
• "The Extra" (1987)
• "Fat Face" (1987)
• "Delivery" (1987)
• "I, Said the Fly" (1989)
• "Salome" (1994)
• "Tollbooth" (1995)
• "Johnny Crack" (1995)
• "Fast Food" (1995)
• "Piece A' Chain" (1996)
• "Water of Life" (1999)
• "For Every Tatter in Its Mortal Dress" (2000)
• "The Rebuke" (2002)
• "The Growlimb" (2004) (Winner of the World Fantasy Award 2005 in the Novella category).
• "The Pool" (2007)
• "Tsathoggua" (2008)
• "The Battery"
• "The Presentation"
• "Copping Squid"
• "Dagoniad"

Reviews

• "Reprisal by Mitchell Smith" (2005)

Awards

Nebula Best Novellette Finalist (1980) : The Angel of Death

Nebula Best Novella Finalist (1981) : The Autopsy

Hugo Best Novellette Finalist (1981) : The Autopsy

Hugo Award Finalist (1981): Polyphemus

World Fantasy Best Novel winner (1983) : Nifft the Lean

World Fantasy Best Collection Finalist (1988) : Polyphemus

World Fantasy Award (2005) Novella winner: The Growlimb.

International Horror Guild Award (2005) Finalist: The Growlimb

British Fantasy Society Award (2005) Finalist: The Growlimb

Horror Writers Association Bram Stoker Awards (2005) Nominee: The Growlimb