Reilly was a Lieutenant in the United States Air Force who went through the Stargate with the rest of the original Stargate team. He stayed with the base camp team when Colonel Jack O'Neill went looking for Dr. Daniel Jackson. While staying at the base, Reilly asked why they could not simply use the Stargate themselves, and Louis Ferretti had to educate him on the Stargate's endless possibilities. When Ra's Horus Guards attacked, Reilly was the first soldier attacked by the guards. Reilly was killed, but for unknown reasons his body was never deposited with the other soldiers. (Stargate)
His fate is never clearly revealed in the movie, and in the book he disappears after the team at the pyramid is captured.