Karnack is a rural unincorporated community in northeastern Harrison County near Caddo Lake in the eastern region of the U.S. state of Texas. The population was 775 at the 2000 census. The town is named after Karnak, Egypt. It was thought that the community's alignment with the city of Port Caddo was relative to that of Karnack and Thebes. The town is the birth place and location of the childhood home of First Lady Lady Bird Johnson. It is also the home of the Longhorn Army Ammunition Plant.