In Harrison County, Karnack is located northeast of Marshall on Hwy 43, on Caddo Lake which straddles the Louisiana border. Its name reportedly derived from the supposition that its distance from Port Caddo (the northeastern port of entry for the Republic of Texas) was the same as that of Karnak from Thebes in ancient Egypt.
Lady Bird Johnson's childhood home, a two-story, white-brick house from the era of antebellum Texas, is at a site 2.5 miles southwest of town on Hwy 43.
Karnack became known from 1989 to 1991 because of the destruction of Pershing IA and II missiles at Longhorn Army Ammunition Plant there.
In 1989 Karnack had twelve businesses, and in 1990 it reported 775 residents.