Pinal County was formed from portions of Maricopa and Pima counties on February 1, 1875, in response to the petition of residents of the upper Gila River Valley, as "Act Number 1" of the Eighth Territorial Legislature. Florence, established in 1866, was designated and has remained the county seat.
The county encompasses 5,374 square miles, of which 4.5 are water. In both economy and geography, Pinal County has two distinct regions. The eastern portion is characterized by mountains with elevations to 6,000 feet and copper mining. The western area is primarily low desert valleys and irrigated agriculture.
Of the 3,439,369 acres that comprise Pinal County, the state of Arizona is the county's largest landholder with 35%, followed by individuals and corporations, 22%; Indian reservations, 23%; the U.S. Forest Service and Bureau of Land Management (BLM), 14%, and the remaining 6% is other public lands.
The Arizona Army National Guard, the United States Department of Homeland Security, the Border Patrol and other federal government agencies all have a presence in Pinal County on multiple federal properties.