The area now known as Fresno County, was discovered by Spaniards during a search for suitable mission sites. In 1846, this area became the property of the United States as a result of the Mexican War.

Fresno County was formed in 1856 from parts of Mariposa, Merced and Tulare counties. Fresno is Spanish for "Ash" and it was due to the abundance of the shrubby local Ash tree, fraxinus dipetala, growing along the San Joaquin River that it received its name.

Parts of Fresno County's territory were given to Mono County in 1861 and to Madera County in 1893.

The original county seat was along the San Joaquin River in Millerton, but was moved to the rapidly growing town of Fresno on the newly built Southern Pacific Railroad line after a flood destroyed much of the town.

Source: Wikipedia