As of the 2010 census, the county had a population of 823,318. The county seat is the city of Ventura (officially known as San Buenaventura). Ventura County's largest city is Oxnard, with a population of about 200,000.
Active occupation of California by Spain began in 1769. Gaspar de Portolà led a military expedition by land from San Diego to Monterey, passing through Ventura County in August of that year. A priest with the expedition, Father Juan Crespí, kept a journal of the trip and noted that the area was ideal for a mission to be established and it was a "good site to which nothing is lacking". Also on this expedition was Father Junípero Serra, who later founded a mission on this site.
On March 31, 1782, the Mission San Buenaventura was founded by Father 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, originally and still officially named San Buenaventura, has been known as Ventura since 1891.
In the 1790s, the Spanish Governor of California began granting land concessions to Spanish Californians, often retiring soldiers. These concessions were known as ranchos and consisted of thousands of acres of land that were used primarily as ranch land for livestock. In Ventura County, Rancho Simi was granted in 1795 and Rancho El Conejo in 1802.
In 1822, California was notified of Mexico's independence from Spain and the Governor of California, the Junta, the military in Monterey and the priests and neophytes at Mission San Buenaventura swore allegiance to Mexico on April 11, 1822. California land that had been vested in the King of Spain was now owned by the nation of Mexico.
By the 1830s, Mission San Buenaventura was in a decline with fewer neophytes joining the mission. The number of cattle owned by the mission dropped from first to fifteenth ranking in the California Missions. The missions were secularized by the Mexican government in 1834. The Mexican governors began granting land rights to Mexican Californians, often retiring soldiers. By 1846, there were 19 rancho grants in Ventura County. In 1836, Mission San Buenaventura was transferred from the Church to a secular administrator. The natives who had been working at the mission gradually left to work on the ranchos. By 1839, only 300 Indians were left at the Mission and it slipped into neglect.
Several outhouses were discovered in July 2007 dating back to the 1800s where a new site had been cleared to prepare for development. The area proved to be a treasure trove for archaeologists who braved the lingering smell in the dirt to uncover artifacts that showed heavy utilization by mission inhabitants, Indians, early settlers and Spanish and Mexican soldiers.
The Mexican–American War began in 1846 but its effect was not felt in Ventura County until 1847. In January of that year, Captain John C. Frémont led the California Battalion into San Buenaventura finding that the Europeans had fled leaving only the Indians in the Mission. Fremont 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.
By 1849, a constitution had been adopted for the California territory. The new Legislature met and divided the pending state into 27 counties. At the time, the area that would become Ventura County was the southern part of Santa Barbara County.
The 1860s brought many changes to the area. A drought caused many of the ranchos to experience financial difficulties and most were divided, sub-divided and sold. Large sections of land were bought by eastern capitalists based on favorable reports of petroleum deposits. A United States Post Office was opened at Mission San Buenaventura in 1861. On April 1, 1866, the town of San Buenaventura was incorporated becoming the first officially recognized town in what would become Ventura County.
On January 1, 1873, Ventura County was officially split from Santa Barbara County, bringing a flurry of change. That same year, a courthouse and wharf were built in San Buenaventura. A bank was opened and the first public library was created. The school system grew, with the first high school opening in 1890.
Other towns were starting in the county. A plan for Hueneme (later Port Hueneme) was recorded in 1874, and Santa Paula's plan was recorded in 1875. The community of Nordhoff (later renamed Ojai) was started in 1874. Bardsdale, Fillmore, Piru and Montalvo were established in 1887. 1892 saw Simi (later Simi Valley), Somis, Saticoy and Moorpark. Oxnard was a late-comer, not being established until 1898.
The Southern Pacific Railroad laid tracks through San Buenaventura in 1887. For convenience in printing their timetables, Southern Pacific shortened San Buenaventura to Ventura. The Post Office soon followed suit. While the city remains officially known as San Buenaventura, it is more commonly referred to as Ventura. The rail line to northern California originally went through Saugus, Fillmore and Santa Paula, providing a boom to those communities along the line. In 1905, Tunnel #26 was completed between Chatsworth and Corriganville near Simi Valley, shortening the rail route. At 7,369 feet, Tunnel #26 was the longest tunnel ever constructed in its day. This tunnel joined the railroad spur coming the other direction from Montalvo through Camarillo, Moorpark and Simi Valley, making the contemporary main line used today. One stop along the way, at a 90 degree turn, was at a sugar beet processing factory. The factory bore the name of its absentee owners, the Oxnard Brothers. A small community of farm and factory workers grew near the train stop. That community, now bearing the name of the factory shortened to the one word train stop, has become the largest city in Ventura County, Oxnard.
Oil has been known in Ventura County since before the arrival of the Europeans, as the native Chumash people used tar from natural seeps as a sealant and waterproofing for baskets and canoes. In the 1860s, several attempts were made to harvest the petroleum products under Ventura County but none were financially successful, and the oil speculators eventually changed from oil to land development. In 1913, oil exploration began in earnest, with Ralph Lloyd obtaining the financial support of veteran oil man Joseph B. Dabney. Their first well, named "Lloyd No. 1", was started on January 20, 1914. The well struck oil at 2558 feet (780 m) but was destroyed when it went wild. Other wells met a similar fate, until 1916, when a deal was struck with the Shell Oil Company. 1916 was the year the large South Mountain Oil Field was discovered, and other deals followed with General Petroleum in 1917 and Associated Oil Company in 1920. 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 of oil a day, with annual production of over a million and a half barrels. More oil fields came on-line in the 1920s and 1930s, with the Rincon field, the second-largest, in 1927, and the adjacent San Miguelito in 1931.
In the early hours of the morning of March 13, 1928, the St. Francis Dam collapsed, sending billions of gallons of water rushing through the Santa Clarita Valley, killing 385 people, destroying 1,240 homes and flooding of land, devastating farm fields and orchards. This was the largest single disaster to strike Ventura County.
Ventura County can be separated into two major parts, East County and West County. East County consists of all cities east of the Conejo Grade. Geographically East County is the end of the Santa Monica Mountains, in which the Conejo Valley is located, and where there is a considerable increase in elevation. Communities which are considered to be in the East County are Thousand Oaks, Newbury Park, Lake Sherwood, Hidden Valley, Santa Rosa Valley, Oak Park, Moorpark, and Simi Valley. A majority of these communities are in the Conejo Valley.
West County, which is everything west of the Conejo Grade, consists of communities such as Camarillo, Oxnard, Somis, Point Mugu, Port Hueneme, Ventura, Ojai, Santa Paula, and Fillmore. West County consists of some of the first developed cities in the county. The largest beach communities are located in West County on the coastline of the Channel Islands Harbor.
Starting in the mid-1900s, there was a large growth in population in the East County, moving from the San Fernando Valley in Los Angeles and out into the Conejo and Simi Valleys, which consists of Calabasas, Hidden Hills, Agoura Hills, Agoura, and parts of Westlake Village belonging to Los Angeles County. The other half of the Conejo Valley, which belongs to Ventura County, consists of Lake Sherwood, Hidden Valley, Oak Park, parts of Westlake Village, Thousand Oaks, and Newbury Park, which was formerly an unincorporated area that is now the most westerly part of Thousand Oaks. Many working-class white people migrated to this area during the 1960s and 1970s out of East and Central Los Angeles. As a result, there was a large growth in population into the Conejo Valley and into Ventura County through the U.S. Route 101 corridor. Making the U.S. 101 a full freeway in the 1960s, and the expansions that followed, helped make commuting to Los Angeles easier and opened the way for development westward. The communities that have seen the most substantial development are Calabasas, Hidden Hills, Agoura Hills, Westlake Village, Thousand Oaks, and Newbury Park.
Development moved farther down the US 101 corridor and sent population rising in West County cities as well. The largest population growth there has been in Camarillo, Oxnard, and Ventura. Development in the East County and along the US 101 corridor is becoming rare today, because most of these cities were master-planned cities, such as Thousand Oaks and Simi Valley, and are approaching build-out. Although the area still has plenty of open space and land, almost all of it was put aside and mandated never to be developed as part of the master plan of each city. Because of this, its private low-key location, its country feel, and its close proximity to Los Angeles, the Conejo Valley area has become a very attractive place to live. It once had relatively inexpensive real estate, but prices have risen sharply. For example, real estate in Newbury Park has increased in price by over 250% in the last 10 years. Median home prices in the Conejo Valley now range from $700,000 to $2.2 million. The Conejo Valley area is one of the most affluent areas in the United States.