Taking its name from the City of Riverside, the county was formed in 1893 from a small portion of San Bernardino County and a larger part of San Diego County.

In the late 18th century, the Spanish mission fathers of San Gabriel (Los Angeles County), San Juan Capistrano (Orange County), and San Luis Rey (San Diego) began colonizing the land and gradually used the interior valley (in what is now Western Riverside County) for raising grain and cattle. During this period, Spain claimed all of California and Mexico.

In 1848, with the signing of the Treaty of Guadalupe Hidalgo, California became a territory of the United States, and in 1850 California became a state. This event generated a steady flow of settlers into the area, including gold miners, entrepreneurs, health-seekers, speculators, politicians, adventurers, seekers of religious freedom, and individuals desiring to create utopian colonies.

In May 1893, voters living within an area carved from San Bernardino County (to the north) and San Diego County (to the south), approved formation of Riverside County. On May 9, 1893, the County officially formed, and began charting a course under its newly-elected Board of Supervisors. The County's early years were linked to the agriculture industry...but commerce, construction, manufacturing, transportation, and tourism soon took hold...contributing substantially to the region's rapid growth.