Pecos County, the second largest county in the state, is in southwest Texas, bordered on the east by Terrell County, on the south by Brewster County, on the west by Jeff Davis and Reeves counties, and on the north by Ward, Crane, and Crockett counties. The center of the county lies at 30°40' north latitude and 102°40' west longitude. Fort Stockton, the county seat, is seventy-five air miles southwest of Odessa. The area is named for the Pecos River, which flows along the northern boundary of the county. Pecos County covers 4,776 square miles and lies primarily in the Trans-Pecos physiographic region, which begins at the Pecos River. In the south and east the county has undulating to hilly terrain surfaced by dark and loamy soils over limestone; this part of the county is 90 percent exposed rock. The northern and western parts of the county have alkaline and loamy desert soils, with some clayey subsoils, over limestone bedrock. Elevations range from 2,500 to 4,000 feet above sea level, with the highest elevations found in the Glass Mountains, in the southwestern part of the county. The eastern and central portions of the county, on the edge of the Edwards Plateau, are marked by mesas of varying sizes with intervening arroyos. Hills become more rounded and valleys more pronounced further west. Most of the northern part of the county slopes toward the Pecos River, while terrain in the western sections is generally undulating. The Pecos River is the only major source of surface water in the county. It flows continuously, while other streams in the county flow only after infrequent torrential rains. Springs were at one time an important water source for the area, but because of heavy pumping of ground water, most major springs no longer flow. The average annual rainfall is thirteen inches. Temperatures range from an average low of 31° F in January to an average high of 96° F in July; the average growing season lasts 225 days. Pecos County land has short and sparse grasses with desert scrubs and cacti. In the southeast, slightly taller grasses, desert shrub, and scrubby live oaks are present. Deer, foxes, ring-tailed cats, sandhill cranes, doves, and ducks are some of the animals that roam the area. The agricultural sector of the county's economy revolves primarily around sheep ranching; cattle, angora goats, and hogs are also raised in the area. Though less than 1 percent of the county is considered prime farmland, crops grown in Pecos County include cantaloupes, carrots, bell peppers, onions, peaches, and pecans. Since the 1920s, however, oil and gas production have dominated the local economy. In 1990 26,650,000 barrels of crude oil were produced in the county; by January 1, 1991, almost 1,505,424,000 barrels of crude had been produced in the area since 1926.
Archeological excavations in Pecos County, at Squawteat Peak, have revealed a large prehistoric camp with numerous artifacts and signs of human occupation. A ring midden in the camp provided a radiocarbon date of 1300 A.D. Archeological finds along Tunas Creek include a burial site, pictographs, and artifacts; a possible modified Langtry projectile point (2,000 B.C. to A.D. 700–800) was found near the creek. The Comanche Trail crossed Pecos County from on or near Horsehead Crossing and through Comanche Springs. Traders on the Chihuahua Trail passed through the area by Comanche Springs about 1840. The first permanent settlement was a United States Army outpost, Fort Stockton, which was established in 1859 at Comanche Springs to guard the San Antonio-El Paso Mail. That same year the Butterfield Overland Mail began service to the army post. The town of St. Gall, later renamed Fort Stockton, was established near the Fort Stockton army post at Comanche Springs by Peter Gallagher, who purchased land for a town site in 1868. St. Gall became a supply center for the army, mail stages, wagon trains, and travelers. One of the first modern attempts at irrigation farming in Texas took place near the settlement in the 1870s.
The land of Pecos County was originally in Bexar Territory and later part of Presidio County. Pecos County was established by the Texas legislature in 1871. The county was formally organized on March 9, 1875, at St. Gall, which became the county seat. There were 1,100 people living in the county that year. The United States agricultural census for 1880 reported 150 ranches and farms in the area. Most holdings were less than ten acres in size, and all but one had less than 500 acres; the average holding was sixty-one acres. About 2,500 acres in the county were planted in corn that year, along with seven acres of oats and twenty acres of wheat. During the 1880s St. Gall was renamed Fort Stockton, and the army post closed, causing a temporary economic slump in the county due to lost trade and employment. The county lost virtually all of its small landholders during the 1880s; in 1883 parts of the county land was split off to form Reeves and Terrell counties, and in 1885 another part was incorporated into the new Val Verde County. By 1890 only twelve ranches, together encompassing 14,564 acres, remained. All but one were larger than 500 acres in size. The county had 227 cattle and 150 sheep that year, and 1,300 acres were planted in corn. By 1900 the area's population had increased to 2,360. There were ninety-five ranches and farms, encompassing 2,159,000 acres, in Pecos County that year; the holdings averaged 22,721 acres in size. The area's economy had become almost completely dominated by cattle and sheep ranching, though plots of wheat, rye, corn, and oats were grown. The agricultural census reported 74,000 cattle and 142,000 sheep in the area that year. By 1910 there were only fifty-six farms and ranches in the county, and the population had declined to 2,071. Ranching dominated the local economy more than ever. Only $200 worth of cereals and vegetables were grown in the county in 1910, but there were 109,000 cattle and 61,000 sheep that year.
Construction of the Kansas City, Mexico and Orient Railway Company of Texas across Pecos County in 1913 caused a boom in land speculation and community growth. Irrigation projects along the Pecos River in the northern part of the county were started to attract land buyers. The communities of Baldridge, Belding, Buena Vista, Chancellor, Girvin, and Imperial were established during this period. By 1920 there were 207 farms and ranches in the area. Almost 8,000 acres was planted in cotton, another 534 was planted in sorghum. The county's developing economy was reflected in its growing population: the number of people living in Pecos County rose from 2,071 in 1910 to 3,058 by 1920. Several unsuccessful attempts to find oil in Pecos County were made after the beginning of the twentieth century, The Yates field was discovered in 1927. It was one of the largest in the nation at the time and caused a boom. By 1930 the towns of Red Barn, Iraan, and Bakersfield had been established to accommodate the influx of prospectors, oil workers, and others searching for opportunities. Meanwhile, the number of farms in the area also continued to grow. By 1930 there were 385 farms and ranches in the county; half of them (51 percent) were operated by tenants. Largely because of the oil boom, but also because of the continuing farm expansion, the population of the county more than doubled during the 1920s, rising to 7,812 by 1930. Farmers in the county were hit hard during the Great Depression of the 1930s; by 1940 only 326 farms and ranches remained in the county. Most of those who left the land were tenant farmers; while there had been 198 tenant farmers in the county in the county in 1939, only 145 remained by 1940. Pecos County's continuing oil production helped to stabilize the local economy, however, as the area was one of the most productive oil counties in the state at that time; in 1938, for example, 15,387,000 barrels of crude were taken from Pecos wells. By 1940 the county's population had increased to 8,185. Construction of paved highways in Pecos County began in the 1930s. By 1931 U.S. Highway 290 and State Highway 82, later U.S. 285 and State Highway 18, were constructed across the county to Fort Stockton. By 1946 U.S. Highway 67, running from Dallas to Presidio across Pecos County, was completed. Meanwhile, the area's oil production continued to increase; in 1948, for example, 22,579,000 barrels were pumped in the county. By 1950 the population of Pecos County had increased to 9,939.
Tourism and new discoveries of petroleum and natural gas helped the economy in Pecos County to grow in the 1950s and 1960s. In 1952 the largest gas field in the county was opened on the M. C. Puckett and Son Ranch, south of Fort Stockton. Tourism was boosted in 1956, when construction of U.S 290 linked Fort Stockton to the Big Bend National Park. The number of people living in the county increased to 11,957 by 1960, to 13,748 by 1970, and to 14,618 by 1980. In the 1980s the economy of Pecos County continued to be based on farming, ranching, oil and gas production, and tourism. In 1982 the county ranked 136th in the state in agricultural receipts; 57 percent of its agricultural income was from livestock and livestock products. In that same year the county produced 274,741,143,000 cubic feet of gas well gas and 50,472,709 barrels of crude oil. Tourism generated 675 jobs with $5,392,000 in payroll. Oil production in Pecos County declined during the 1980s, however, dropping to 31,994,000 barrels by 1989. The voters of Pecos County supported the Democratic ticket in virtually every presidential election between 1872 and 1948; the only exception occurred in 1920, when they supported Republican Warren G. Harding. The county's voters gave majorities to Republican Dwight D. Eisenhower in 1952 and 1956, then swung back to the Democrats in 1960, 1964, and 1968. In every presidential election between 1972 and 1992 the county went Republican. By 2014 there were 15,893 people living in Pecos County. About 27 percent were Anglo, 4.3 percent African American, and 67.5 percent Hispanic. The largest communities in the county are Fort Stockton (population, 8,547), the county seat and petroleum distribution center, and Iraan (1,273), an oil and gas center and the birthplace of the Alley Oop comic strip. Other communities include Sheffield (322), Coyanosa (174), Girvin (20), Imperial, and Bakersfield.