Tucson stands on what archaeologists believe is the longest continuously inhabited site in the nation. Humans came to the area as early as 11,000 years ago. The Hohokam ("the vanished ones") Indians built a sophisticated system of irrigation canals to farm their fields in the first century, when a pit house was built where downtown Tucson now stands. Then they inexplicably disappeared, later to be replaced by Pima and Tohono O'odham tribes. Italian Father Eusebio Francisco Kino, who served as a missionary for the Spanish church, made his initial visit in 1694 to what the Pima Indians called "Stjukshon", meaning, roughly, "spring at the foot of a black mountain." The word referred to the then-flowing Santa Cruz River.

Several centuries later, in 1775 (a year before American Independence), Spanish soldiers established a fort they called San Augustin del Tucson at the foot of the mountain, which is now officially called Sentinel Peak. The city of Tucson was founded August 20, 1775, by Irishman Hugh O'Connor, more notably known as Hugo O'conor for his explorations for Spain. A walled presidio provided refuge for travelers and residents and was soon nicknamed the "Old Pueblo," and endearment still used today.

Spain's claim to Tucson ended when Mexico gained independence in 1821. Tucson became part of the United States with the Gadsen purchase in 1853 and remained so except for a brief period when Confederate soldiers seized th3e city during the Civil War.
Tucson was known as a rowdy frontier town tempered by social refinements brought by city settlers. In 1867, when the Old Pueblo was named capital of the newly formed Arizona Territory, the town's people invested in what they hoped would become a stable community. The capital was moved north before Arizona became a state in 1912. Tucson ultimately was allowed to establish another prestigious state institution, the University of Arizona.