Harness' first story, "Time Trap" (1948), is unusual for a first story in that it shows many of his recurring themes, among them art, time travel, and a hero undergoing a quasi-transcendental experience.

Harness' most famous single novel was his first, Flight into Yesterday, which was published first as a novella in the May 1949 issue of Startling Stories and was later republished as The Paradox Men in 1953. The "science-fiction classic" is both "a tale dominated by space-opera extravagances" and "a severely articulate narrative analysis of the implications of Arnold J. Toynbee's A Study of History." In his introduction in the 1967 Four Square paperback reprint of the novel, Brian Aldiss terms it a major example of the "Widescreen Baroque" style in science fiction, and John Clute terms it "the kind of tale which transforms traditional space opera into an arena where a vast array of characters can act their hearts out, where anything can be said with a wink or dead seriously, and any kind of story be told." The Paradox Men features the concept of force field which protect people against high-velocity weapons like guns but not against knives or swords, an idea later used in Frank Herbert's Dune (1965).

During 1953, Harness also published his most famous single story, "The Rose", which was published first in the British magazine Authentic Science Fiction, then as the main novella in a UK mass-market paperback collection. The story did not appear in the United States until 1969.

Among Harness' best known stories are "The Rose", "An Ornament to his Profession", "The Alchemist" and "Stalemate in Time". His story "The New Reality" has been called "SF's best Adam & Eve story" by Brian Stableford. His novel Redworld is one of the very few science fiction novels for which all characters are aliens.

Harness's ideas influenced numerous writers and he continued to publish until 2001, being nominated for multiple Hugo and Nebula awards. During 2004 he was named Author Emeritus by the Science Fiction and Fantasy Writers of America.

Harness died in 2005, at the age of 89, in North Newton, Kansas.

Novels

• Flight into Yesterday (1953)(reprinted as The Paradox Men in 1955)
• The Ring of Ritornel (1968)
• Wolfhead (1978)
• The Catalyst (1980)
• Firebird (1981)
• The Venetian Court (1982)
• Redworld (1986)
• Krono (1988)
• Lurid Dreams (1990)
• Lunar Justice (1991)
• Drunkard's Endgame (1999) (in Rings, an omnibus edition of four novels by Harness from NESFA Press ISBN 1-886778-16-7)
• Cybele, With Bluebonnets (2002)

Collections

• The Rose (1966)
• An Ornament to His Profession (1998) (NESFA Press reprint collection, ISBN 1-886778-09-4)