Back then, Riverside County was inhabited by different tribes of Indians such as the Cupeño, Luiseño, and Cahuilla. The Mission San Luis Rey de Francia estancia was the first European settlers of this county. In 1819, Leandro Serrano was permitted to occupy the land and use it for farming and grazing. In 1833, Mexico confiscated several Mission lands. Afterward, several rancho land grants were executed throughout the state as well as in Riverside County.

In 1850, 27 California counties were recognized. The area of Riverside County now was divided into two parts, San Diego County and Los Angeles County. The eastern part of Los Angeles County became the location of San Bernardino County in 1853. There were several proposals for creating new counties in Southern California, however, none were made. On March 11, 1893, Governor Henry H. Markham approved the creation of Riverside County.

The county is composed of portions of San Diego County and San Bernardino County. The city of Riverside was chosen as the county seat. It was on May 9, 1893 when Riverside County was officially established. One of the oldest airfields in the U.S. which is the March Air Reserve Base is found in this county. Later on, it was renamed to March Field. In 1947, it was renamed again to March Air Force Base and became an essential Strategic Air Command (SAC) during the Cold War.

Riverside County became the focus of Civil Rights Movements in the United States, specifically the Mexican-American communities of the Coachella Valley and the African-American people of Riverside. Additionally, this county has also been the center of Native American Gaming corporations. In the 1980s, the government wanted to close down small bingo halls operated by the Cabazon Band of Mission Indians and the Morongo Band of Cahuilla Mission Indians. However, the tribes fought back and the Supreme Court ruled in their favor. As a result, these Indian tribes are operating big casinos in the county.

Riverside County was named after its county seat which is the city of Riverside. Since the city was located near the Santa Ana River, it was named as the city of Riverside. The city was founded in 1870, while Riverside County was created in 1893. Riverside County comprises a larger portion of San Diego County and a small part of San Bernardino County.

Its political system started in 1893, but even before that, the land was already occupied by Native American tribes. In 1774, an overland expedition was led by Juan Bautista de Anza. During the 18th century, the Spanish mission fathers colonized the land. The Mexicans were freed from Spanish rule in the 1820s. The lands were then transferred to Californians as grants. The first land grant that was given was Riverside County.

In 1848, California became a part of the United States, upon the signing of the Treaty of Guadalupe Hidalgo. Because of this more and more settlers came to the area. Some of them are adventurers, politicians, entrepreneurs, gold miners, speculators, etc.

In May 1893, some parts of San Diego County and San Bernardino County were formed as Riverside County. The county was officially formed on May 9, 1893 and the Board of Supervisors were elected. During the early years, agriculture was the major industry. But as time passed by, manufacturing, commerce, construction, tourism, and transportation were introduced, contributing to the rapid development of the region.