San Augustine is the county seat city of San Augustine County, Texas, in East Texas, United States. The population was 1,920 at the 2020 ce…San Augustine is the county seat city of San Augustine County, Texas, in East Texas, United States. The population was 1,920 at the 2020 census. The first European settlement in the area began in 1717 with the establishment of Mission Nuestra Señora de los Dolores de los Ais by Spanish missionaries. The mission was abandoned and reestablished in 1721.