San Diego is a small town of 4500 located in the South Texas Plains Region of Southern Texas along Texas Highway 44. San Diego is the county seat of Duval County. Duval County was founded in 1858 and organized in 1879. Early settlers came from Mexico then Anglo settlers arrived in the 1860s. The railroad brought added prosperity to the area. The economy of Duval County was aided further by the discovery of oil, gas, and uranium in the early 1900s.