He is best known for the Hugo and Nebula Award winning novella "Enemy Mine", which was subsequently made into an identically titled movie and a novelization in collaboration with David Gerrold. The story tells of an encounter between a human and an alien soldier, whose races are in a state of war. They are marooned together in space and have to come to grips with the universal problem of facing and accepting xenophobia. A greatly expanded version of the original novella as well as two novels completing the trilogy, The Tomorrow Testament and The Last Enemy are gathered with additional materials into The Enemy Papers.
The original novella, in part, helped Longyear to win the John W. Campbell Award for best new writer for 1980. He is the only writer to win the Hugo, the Nebula, and the Campbell in the same year. (Contrast the other SF "triple crown" winner: William Gibson with the Hugo, Nebula, and Philip K. Dick Award in 1984.)
He also wrote the Circus World and Infinity Hold series, several stand-alone novels, numerous short stories, and two books for the Alien Nation novelisation series. His trilogy "Infinity Hold," "Kill All the Lawyers," and "Keep the Law," was released in 2002 in a single paperback volume by the Author's Guild in a Backinprint.com edition. His recent Jaggers & Shad mystery stories, featuring two detectives in the Artificial Beings Crimes Division (Devon Office)are set mostly in Exeter and the surrounding Devon countryside and villages. The first of the tales, "The Good Kill" won Analog magazine's AnLab award for Best Novella in 2006 and "Murder In Parliament Street" won the same award for 2007.