Yuma County, Arizona: History
Long settled by Native Americans of indigenous cultures for thousands of years, this area was controlled by the Spanish Empire in the colonial era. In the 19th century, it was part of independent Mexico before the Mexican–American War and Gadsden Purchase.
Yuma County was one of four original Arizona counties created by the 1st Arizona Territorial Legislature. The county territory was defined as being west of longitude 113° 20' and south of the Bill Williams River. Its original boundaries remained the same until 1982, when La Paz County was created from its northern half.
The original county seat was the city of La Paz; in 1871 it was moved to Arizona City, later renamed as Yuma in 1873.