San Diego, the county seat of Duval County, is on San Diego Creek and the Texas-Mexican Railway at the intersection of State highways 44 and 359 and Farm Road 1329, sixteen miles northeast of Benavides, twenty-four miles southeast of Freer, and fifty-two miles west of Corpus Christi on the county line between eastern Duval and western Jim Wells counties. The San Diego Cemetery lies northeast of town. Long before the town itself existed, its site was known to transients. In the eighteenth century travelers between Goliad and Mier used the springs that help feed San Diego Creek and now lie within the city limits as a watering hole. Around 1800 San Diego de Arriba and San Diego de Abajo, two grants totaling eight leagues of land, were granted by the Spanish government to Julián Flores and his son Ventura. The grants were surveyed in 1806 by José Faustino Contreras, the surveyor general of San Luis Potosí. Julián and Ventura Flores arrived in 1809 and received their deed in 1812. The first settlers may have been Julián Flores's herdsmen, who had settled on his ranch there by 1815; four years later the Flores family authorized an agent to found a town "at the place called San Diego." In 1828 Luis Muñiz became the first recorded birth in Duval County; he lived until the mid-1840s. By 1844 a visiting surveyor placed "some twenty-five families" there, and in 1846 Gen. Zachary Taylor and his troops reportedly camped there on their way to occupy Port Isabel, but not until 1848, when Henry L. Kinney and William L. Cazneau cut a road from Corpus Christi to Laredo that passed through the area, was the settlement named. In that year Ventura Flores sold some land along the north bank of San Diego Creek to Pablo Pérez, who built some stone houses and brought some families to live there. The resulting community was known as Perezville.
Some sources report that Perezville was renamed San Diego in 1852, when the town's first post office was established in the Casa Blanca, a blockhouse built of handcut caliche on the west bank of San Diego Creek. During the Civil War the building was reportedly used by Confederate troops under Nathan Bedford Forrest, and it was later reincarnated as a general store, a Prohibition era speakeasy, and a private home. The Casa Blanca Bar was still in operation in 1988. Other sources date the first post office from 1867, with George B. Warden as postmaster. At any rate the community remained virtually completely Mexican American until the latter year, when a boom in the sheep industry began attracting several white settlers, who later rose to local prominence. Among them were James O. Luby, Walter W. Meek, Sr., Norman G. Collins, Capt. E. N. Gray, Frank C. Gravis, Frank W. Shaeffer, and William Hubberd. One other event of lasting significance for the town occurred in 1867: Father Claude Jaillet built a ten-by-thirty-foot wooden church, which became the only public place of worship between Corpus Christi and the Rio Grande.
For the next fifteen years or so, while the price of wool remained high, San Diego grew and prospered. In 1875, when the population reportedly consisted of "about 500 Mexicans and 50 Americans," Luby, Shaeffer, and two other men were appointed to partition the town, and the sale of town lots began shortly thereafter. A year later, when Duval County was organized, San Diego was chosen as the county seat. The town was still far from genteel; in 1878 2,000 federal troops were stationed in San Diego to afford the citizens some protection from Mexican and Indian raiders, and in 1879, during a period of protracted drought, five men were shot dead in downtown San Diego as part of a dispute over a local waterhole. In 1888 the county election campaign between the Botas and Guaraches (Democrats and Republicans) turned ugly when Francisco de Gonzales, the newspaper editor and guarache partisan, angrily confronted a group of boys who had been following a wagon of bota musicians. On the following day Anastacio Gómez, the father of one of the boys, sought out Gonzales in a local barbershop; the two exchanged angry words, whereupon Gonzales pulled a gun and shot Gómez, who died soon thereafter. Also in 1879 the Corpus Christi, San Diego and Rio Grande Railroad reached the town, making the port of Corpus Christi easily accessible for Duval County ranchers and farmers. Two years later the Texas-Mexican Railway took over the Corpus Christi, San Diego and Rio Grande and then built on to Laredo, completing the first rail link between Corpus Christi and northern Mexico. With this extension of the railroad San Diego's importance as the county's leading agricultural shipping center grew. It retained that status after cotton began to replace wool as Duval County's most important agricultural product in the early 1880s.
Despite the influx of white settlers, San Diego remained in many ways more Mexican than American. Bullfights provided the usual Sunday afternoon entertainment, and Mexican Americans still played a prominent role in San Diego's affairs. Encarnación García Pérez was a leading local farmer who grew cotton and onions and raised sheep. Fabián Favela, the tailor, also tried his hand at hat manufacturing and in 1886 reportedly collected money to buy cottonseed for distribution among the poor. During the 1880s Luis Puebla, a graduate of Georgetown University in Washington, D.C., and a former professor at Lagarto College in Live Oak County, directed the town's public school system, and the Methodist minister Santiago Tafolla preached regularly in San Diego. In 1882 Francisco P. de Gonzales founded El Eco Liberal, the first newspaper in Duval County, and two years later another newspaper editor, Salvador R. de la Vega, imported a Mexican theatrical troupe to perform for the benefit of a proposed San Diego College. At the time, more than sixty Duval County young people were pursuing their studies elsewhere, and the college was intended to help keep some of these scholars at home. It never got off the ground, but symbolizes the determination and optimism that prevailed in San Diego in the late nineteenth century. In 1884 the town had three churches, a bank, and an estimated population of 1,000. By 1890 it had a population of 1,500, four churches, and two cotton gins. The town continued to grow rapidly in the early twentieth century. During the 1906–07 school year 422 pupils attended San Diego's public school, which also employed six teachers and was by far the largest in Duval County.
Unfortunately, San Diego's tradition of violence, usually politically motivated, also persisted into the twentieth century. On December 20, 1907, shortly after engineering a sweeping guarache triumph in the county elections, John Cleary, the county tax assessor and leader of the Duval County Democratic Party, was killed by a shotgun blast while sitting in a San Diego restaurant. His assassin escaped while local lawmen were enjoying a fiesta, which they claimed had drowned out the sound of the shooting. Two Texas Rangers arrived on the following morning to take charge of the investigation, and in April 1908 they arrested San Diego merchant T. J. Lawson, his son Jeff, and a former deputy sheriff named Candelario Saenz, who was identified as the triggerman. The grand jury indicted Saenz and dismissed the evidence against the Lawsons as circumstantial, but before the trial two witnesses died of natural causes and Saenz went free. Cleary's assassination opened the Duval County Democratic leadership to Archer Parr, thereby beginning perhaps the most remarkable political dynasty in Texas (see BOSS RULE).
Ironically, Saenz was shot dead in a political dispute four years later. C. M. Robinson, the county Democratic chairman, had fomented a revolution against Parr's despotic control of local politics. Parr had arranged a referendum on the question of incorporating San Diego, no doubt hoping to increase the number of patronage jobs at his disposal, for May 18, 1912. On that morning Robinson and three of his allies encountered three Mexican-American county officials, including Saenz, who were Parr loyalists. Insults and then shots were exchanged, and all three Mexican Americans were killed. The Parr forces claimed that the Robinson camp had provoked the gunplay to intimidate Mexican Americans into staying home on election day, and in fact only about 200 people voted, and the incorporation measure failed by a two-to-one margin. Robinson himself had been unarmed, but the sheriff of Jim Wells County quickly arrested his associates and transferred them to Corpus Christi. The three went free on bond and were finally acquitted in April 1914. Meanwhile the town tried to get on with business as usual. By the time of the acquittal a San Diego Commercial Club had been founded, and among the town's commercial establishments were three banks, three cotton gins, an automobile livery, and a brewery. A former employee of the latter concern played a key role in perhaps the most bizarre episode in San Diego's history, and one that symbolized the depth of the town's identification with Mexico. In January 1915 a Hidalgo County deputy sheriff arrested Basilio Ramos, Jr., in McAllen. Ramos, who had recently quit his job with the Royal Brewing Company, was carrying a copy of what became known as the Plan of San Diego.
San Diego's estimated population fluctuated between 2,500 and 3,500 during the 1920s and 1930s, but by the early 1940s the town was reported to have 2,674 residents and 110 businesses. In 1938 a pharmaceutical company and a tile manufacturer were located there, and in the mid-1950s Manuel Marroquín was producing corn tortillas, but despite the presence of Archie Parr's son and successor, George B. Parr, who made San Diego his home, the town lacked the industrial clout of Freer, whose economic prosperity was based on oil. San Diego held an annual Fiesta of Progress from 1946 to 1950 to celebrate its prosperity, but it was discontinued in 1951 due to economic reverses caused by a prolonged drought. In 1950, when the estimated population had grown to 4,394, the eighty-five retail and ten service establishments in San Diego had combined sales of $2,160,000. In 1954 the Fiesta of Progress was revived to help pay for a new community center, built as a memorial to the town's World War II dead, but in 1955 the town built a new little league baseball field on the old fiesta grounds. In 1990 the population of San Diego was 4,983.