Karnack, Texas is located in in northeastern Harrison County. Established in 1898, the townsite was named for the supposition that its distance from Port Caddo was the same as that of Karnak from Thebes in ancient Egypt. In 1898 a post office was granted, and by 1906 Caddo Lake oilfields began producing.

Lady Bird Johnson was born in Karnack in a white two-story house from the Antebellum era, approximately 2.5 miles southwest of town.