Ventura County was historically inhabited by the Chumash people, who also settled much of Santa Barbara and San Luis Obispo Counties, with their presence dating back 10,000-12,000 years.

In October 1542, the expedition led by Juan Rodríguez Cabrillo anchored in an inlet near Point Mugu. Its members were the first Europeans to arrive in the area that would become Ventura County.

Active occupation of California by Spain began in 1769. Gaspar de Portolá led a military expedition by land from San Diego to Monterey, passing through Ventura County in August of that year.

On March 31, 1782, the Mission San Buenaventura was founded by Father Junípero Serra. It is named after Saint Bonaventure, one of the early intellectual founders of the Franciscan Order. The town that grew up around the mission, was originally and remains named San Buenaventura, although has been known as Ventura since 1891.

In 1822, California was notified of Mexico's independence from Spain. The priests and neophytes at Mission San Buenaventura swore allegiance to Mexico.

In 1847, Captain John C. Frémont led the California Battalion into San Buenaventura finding that the Europeans had fled leaving only the Indians in the mission. Frémont and the Battalion continued south to sign the Treaty of Cahuenga with General Andrés Pico. The Treaty of Guadalupe Hidalgo formally transferred California to the United States in 1848.

On January 1, 1873, Ventura County split from Santa Barbara County and the city of Ventura became the county seat.

In 1887, the Southern Pacific Railroad laid tracks through San Buenaventura. For convenience in printing timetables, Southern Pacific shortened San Buenaventura to Ventura. The Post Office soon followed suit.

Oil had been known in Ventura County since before the arrival of the Europeans. Oil exploration began in earnest in 1913. At its peak, the largest oil field in the county, the Ventura Avenue oilfield, discovered in 1919 in the hills north of Ventura, was producing 90,000 barrels of oil a day.

In 2016, the United States military was the largest employer in the county with more than 16,000 employees. (Port Hueneme is California’s smallest but only deep water port between Los Angeles and San Francisco.) The County of Ventura was the next largest employer with nearly 8,000 employees located throughout the county.