His father was a tribal chief on Mordan and was assassinated in 2319 by Peretor Sain. In response, Karnas took hostage 63 passengers aboard a starliner and demanded weapons from Starfleet in exchange for their release. He intended to use the weapons to crush his enemies and take control of Mordan. Admiral Mark Jameson was sent to negotiate the hostages' release, but instead supplied weapons to Karnas, unknown to Starfleet. In order to even things up, Jameson supplied weapons to Karnas' rival as well. This plunged Mordan into forty years of civil war.
Karnas emerged victorious and become leader of Mordan. In 2364 he lured Jameson to Mordan by taking the Federation ambassador hostage. He wanted Jameson's life in return for the hostages, but Jameson died as a result of a de-aging drug he had taken before Karnas could kill him. On seeing the suffering of Jameson, Karnas relented and released the hostages. He also allowed Jameson to be buried on Mordan. (TNG: "Too Short a Season")