In the Mirror Universe, Ensign Kelby was, like his counterpart, a member of Enterprise's engineering crew. He escaped the ship's destruction in a Tholian web via escape pod, and was assigned by Commander Tucker to help get the Defiant's warp engines back online. When engine components under his supervision began to disappear, Tucker told Kelby to find them, or he would tell Commander Archer that Kelby was the reason they did not yet have warp power. Kelby entered a junction in search of the components and was killed by the culprit, a Gorn. (ENT: "In a Mirror, Darkly, Part II")
- The mirror version of Kelby was played by actor Derek Magyar, who also portrays his counterpart.