Pima County, the second-largest of the four original counties, was created in 1864 and included approximately all of southern Arizona acquired from Mexico by the Gadsen Purchase. Settlement of the region goes back to the arrival in the 1690s of the Spanish who encountered Native Americans already living there. About the middle of the 18th century, silver and gold were discovered and prospectors from Mexico entered the area in droves. The latter part of the century saw expansion of mining and ranching in Pima County and an increase in population, despite the threat of attack from Apaches.

The Royal Presidio de San Augustín del Tucson was completed by 1781, and it remained the northern-most outpost of Mexico until the Gadsden Purchase transferred the land to the United States in 1854. Soldiers later arrived in 1856 and the population began to rise. From a population of 395 in 1820, Tucson has grown to be the second largest city in Arizona. It has always served as the Pima County seat and was the Arizona Territorial capital from 1867 to 1877. Tucson is home to the University of Arizona and offers many historical and cultural attractions.

Just south of Tucson is the Mission San Xavier del Bac, founded in 1697 by Father Kino and still in use today. Within Pima County are two cactus forests – Saguaro National Park to the northeast and Organ Pipe Cactus National Monument in the southwestern portion.

Although greatly reduced from its original size, Pima County still covers 9,184 square miles. It ranges in elevation from 1,200 feet to the 9,185-foot peak of Mount Lemmon. The San Xavier, Pascua Yaqui and Tohono O’odham reservations together account for ownership of 42 percent of land located in Pima County. The state of Arizona owns 15 percent; the U.S. Forest Service and Bureau of Land Management, 12 percent; other public lands, 17 percent; and individual or corporate ownership, 14 percent.