The Yokuts and Mono people were the first to occupy the area that is now known as Fresno County. Later, the area was settled by the Spaniards. In 1846, the area became part of the United States after the Mexican War. In 1856, Fresno County was formed, including parts of Merced, Mariposa, and Tulare counties. Some parts of Fresno County were given to Mono County in 1861 and to Madera County in 1893. As Fresno County continued to grow, the county seat, which was in Millerton, was quickly moved to the town of Fresno.
The growth of Fresno County was not easy at all. It faced many adversities that slowed down its economic growth. These adversities include land disputes, civil conflicts, and natural disasters. In 1882, a devastating fire burned down an entire block in the city of Fresno, and in 1883, it happened again. Floods also caused damage in Fresno County during its early days.
Residents of Fresno County first took up agriculture as a source of income. They developed canals that were used for irrigation in farms. These canals were useful in cultivating wheat, which was the major crop in Fresno County for many years. In the 1900s, oil was discovered in the western part of Fresno County near the town of Coalinga. The discovery of oil brought further economic success. By 1910, Coalinga Oil Field was California's most productive oil field. The Coalinga Oil Field continues to produce oil to this date and is currently the eighth-largest field in the State.

Fresno County is named after Fresno Creek. In Spanish, Fresno means “ash tree,” and because of the presence of mountain ash or ash tree Fraxinus dipetala in great abundance along the San Joaquin River in the county, it received its name. The name Fresno was given to the county by the Spaniards, the first European settlers to arrive in that area. The ash tree is a deciduous shrub or small tree that can grow up to 7 meters tall. The leaves are about 5 to 19 centimeters long, are light to dark green, and the leaflets are about 1 to 7 centimeters long and are serrated along the margins. The flowers of an ash tree are white, have lobe-shaped petals that are 2 to 4 millimeters long, and have a sweet scent.