Pima County lies at the northern range of the Sonoran Desert, and is comprised of mountain ranges, river valleys, and cactus forests. Recent archaeological digs along the Santa Cruz River near Tucson suggest that this area is one of the oldest continuously inhabited areas of the United States, with irrigation canals that may be the oldest in North America. Native Americans have continuously inhabited this region from prehistoric times to the present. The Tohono O'odham reservation in Pima County is the second largest reservation in the nation.
Father Kino founded the Mission of San Xavier del Bac in 1697 and it is still in use today. It is considered by many to be the most beautiful of all the Kino Missions.
In 1775, Juan Bautista de Anza and his colonizing expedition, traveled northward along the Santa Cruz River on their way to San Francisco. Along the way, they passed through a Tohono O’odham settlement they called Tuquison.
Earlier that year, on August 20, 1775, the Spanish authorities founded the Presidio of San Agustín del Tucson, on the banks of the Santa Cruz River. The Royal Presidio de San Augustin del Tucson was completed by 1781, and it remained the northern-most outpost of Mexico until the arrival of American soldiers in 1856. 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 many historical, ecological, and cultural attractions.
The United States
Pima County, the second largest of the four original Arizona counties, was created by the 1st Arizona Territorial Legislature in 1864 with land acquired through the Gadsden Purchase from Mexico in 1853; approximately all of southern Arizona was acquired from Mexico by the Gadsden Purchase. European settlement of the region goes back to the arrival in the 1690s of the Spanish.
The original county consisted of all of Arizona Territory east of latitude 113° 20' and south of the Gila River. Soon thereafter, the counties of Cochise, Graham and Santa Cruz were carved from the original Pima County.
About the middle of the 18th century, silver and gold were discovered in the region 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 roaming bands of Apaches.
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 42.1 percent of county land. The state of Arizona owns 14.9 percent; the U.S. Forest Service and Bureau of Land Management, 12.1 percent; other public lands, 17.1 percent; and individual or corporate ownership, 13.8 percent. Pima County has two Enterprise Zones, one in South Tucson and portions of Tucson and the other in an unincorporated portion of the county just southwest of Tucson.
For more information see Historic Summary of Pima County (PDF)