The Old San Antonio Road was also known as the Camino Real, the King's Highway, and the San Antonio-Nacogdoches Road. The blazing of the trail came about as the result of three expeditions. In 1690 Alonso De León led his fourth expedition into Texas, this time with the objective to establish San Francisco de los Tejas Mission in East Texas (in the future Houston County). In 1691 Domingo Terán de los Ríos, first provincial governor of Texas, crossed the Rio Grande taking additional missionaries to the East Texas missions. Up to the Río Hondo, Terán followed much the same course as traveled by De León. During the journey he diverted his path to send a party to Matagorda Bay to meet supply ships. In 1693 Gregorio de Salinas Varona became the first man to proceed directly from the Rio Grande to the East Texas missions, in an expedition to bring relief supplies; he thus further defined the course of the road as a direct route from Monclova, then capital of the province, to the Spanish missions. In 1714 Louis Juchereau de St. Denis probably followed at least part of the road from Natchitoches to San Juan Bautista on the Rio Grande. In Spanish Texas the Old San Antonio Road was a major artery for travel into Texas. It served as a lifeline for the missions by enabling the transport of freight supplies and military protection, and it facilitated trade. During the eighteenth century Spanish ranchers conducted cattle drives along the route from points in Texas to the annual fair in Saltillo, Coahuila. In addition to being an avenue of commerce, the road enabled immigration. Moses Austin traversed the trail en route to San Antonio to request an empresario grant from the Spanish government in 1820, and many Anglo-American colonists entered Texas at Gaines Ferry on the Sabine and arrived at Nacogdoches and the interior of Texas over the road.
Although generally thought of as a single road, it may be more accurate to describe the Old San Antonio Road as a network of trails, with different routes used at different times. Feeder roads branched off the main course. Considerable portions of the route probably existed before the Spanish arrived. Some of the earliest segments of the road were previously links between Indian settlements. Other segments of the road did not appear until much later. The portion connecting Bastrop and Crockett, for example, did not come into use until after 1790, relatively late in the road's history. During the course of the eighteenth century the route between the Rio Grande and San Antonio was gradually shifted southeastward, probably as a result of the Apache and Comanche threat to Spanish travelers. Nor was the Old San Antonio Road the only camino real in the Provincias Internas. Several routes in what is now the United States were called by that name. The mission trail up the coast of California and the road from El Paso to Santa Fe and Taos were also known as caminos reales.
In 1915 the Texas legislature appropriated $5,000 to survey and mark the route. The Daughters of the American Revolution and other patriotic organizations sponsored and endorsed the project, and professional surveyor V. N. Zivley was commissioned to make the study. In attempting to determine the route Zivley examined river crossings and other topographic features, Spanish land grants, and laws of the Republic of Texas. From the Rio Grande to San Antonio he found little physical evidence of the trail, but by use of Juan Agustín Morfi's diary of 1778, he sought to establish the route. The route traced by Zivley followed a southeasterly course. It began at Paso de Francia on the Rio Grande, passed near Cotulla and Poteet, and entered San Antonio, from where it passed between Hays and Caldwell counties and through Bastrop, Lee, and Burleson counties, formed the boundary between Robertson and Brazos and Madison and Leon counties, and passed through Houston, Cherokee, Nacogdoches, San Augustine, and Sabine counties, before crossing the Sabine River at Gaines Ferry. The total distance was 540 miles. In 1929 the Texas legislature designated the Zivley version of the Old San Antonio Road one of the historic trails of Texas. The legislature also directed the highway department to preserve and maintain the road along the route. Save for some temporary deviations and a few locations impractical for a usable road, most of the distance from the Sabine to San Antonio had been opened and paved by 1949. Much of this route is still in use as State Highway 21 and related country roads.
In order to commemorate the route's 300th anniversary in 1991, the legislature authorized the formation of a nine-member Old San Antonio Road Preservation Commission and a supporting advisory committee. It also directed the State Department of Highways and Public Transportation to identify the disposition of the historical trail, to develop a historic preservation plan, and to prepare a comprehensive report. After a yearlong study that drew on a wide variety of historical and archeological evidence, the project staff, headed by archeologist A. Joachim McGraw, determined that the route that Zivley plotted was only one of several changing historic routes known as the Old San Antonio Road or El Camino Real. The study identified no fewer than five different main routes that were used at various times. Researchers determined that the route employed by travelers often depended on season, natural conditions, and Indians. All of the routes began at the Presidio del Río Grande, also known as San Juan Bautista, which was located at Guerrero, Coahuila, five miles from the Rio Grande and approximately thirty-five miles southeast of the site of present-day Eagle Pass. The presidio served as a gateway for expeditions going into Texas. The routes led across South Texas and converged in San Antonio. These trails, in order of usage, became known as the camino pita, the Lower Presidio road or camino en medio, and the Upper Presidio Road. Zivley's route through South Texas retraced the Lower Presidio Road, in use from approximately 1750 to 1800. Northeastward, beyond San Antonio the roads diverged again. An upper, early trail known as the camino de los tejas followed the springs of the Balcones Escarpment and eventually turned eastward toward the Sabine River. It was used to establish and supply the first eighteenth-century Spanish missions of East Texas. A later road that Stephen F. Austin called the camino arriba was established near the end of the eighteenth century, and though it still led to destinations in East Texas, the route looped southward through a dense post oak savannah.
After Texas independence the road fell into disuse as the greater emphasis was on north-south routes. Courses shifted to accommodate the growth of new settlements and new markets, and to provide access to coastal trade. Shortly after the Mexican War, the camino arriba, what is now called the Old San Antonio Road, regained some of its former importance as travelers from East Texas hastened to San Antonio and onward toward the West Coast during the Gold Rush. Later, during the Civil War, the road served as a significant route for transportation of cotton from eastern Texas to San Antonio to Laredo and on to Mexico. After the war large segments of the route were abandoned in favor of newer, shorter roads that linked the growing urban centers of the state. In the latter part of the nineteenth century the coming of the railroads significantly changed transportation routes and the development of towns and commerce, thereby causing the abandonment of some of the old roads. The remaining portions of the caminos served local transportation needs. After the 1991 highway department study, the state adopted a comprehensive preservation plan, which called for continued efforts to study the road, to preserve existing artifacts, and to develop educational and tourist materials to publicize the road's history.