| Unknown date | Chumashan tribes settled in the area now encompassed by Ventura County. Villages extended from present-day Pt. Mugu to Santa Barbara. |
| 1542 | Cabrillo, the Spanish discoverer of California, sailed up the California coast. He stopped at the Pueblo de las Canoas, presumed to be in Ventura County, possibly where the Ventura River meets the Pacific Ocean. |
| 1602, December | Sebastian Vizcaino's expedition passed in the vicinity of the Channel Islands. |
| 1769, August 10 | Captain Gaspar de Portola and his men left their campsite near Castaic. He spent several days camped near Indian villages in Ventura County, camping near Rincon Point on August 16. |
| 1770, January 12 | Portola's men crossed Ventura County, following the route of the present Highway US101, over the Conejo Grade and through Calabasas. |
| 1774, April 10 | Juan Bautista de Anza camped near Triunfo, in Russell Valley west of Calabasas. The next night, Anza and his party camped near San Buenaventura, on the river of the same name. |
| 1776 | Anza passed through Ventura County with the first overland emigrant trains to California. His party camped near El Rio on February, 23 1776. |
| 1782, March 29 | Father Serra and his party arrived at La Assumpta, the name he gave to the present-day city of San Buenaventura. |
| 1782, March 31 | Father Serra dedicated the San Buenaventura Mission and construction began. |
| 1809, September 9 | The mission church was dedicated. The first liturgical service was held on September 10. |
| 1795 | The first Spanish land grant, Rancho Simi, was made to Patricio, Javier, and Miguel Pico. |
| 1802-3 | The second Spanish land grant, El Conejo, was made to Ygnacio Rodriguez and José Polanco. |
| 1812, December 12 | A series of earthquakes began, damaging the mission church and other structures. |
| 1822 | Californians were notified of Mexico's successful revolt against Spain and California became part of the Mexican Empire. |
| 1822 | Rancho Simi was sold to Jose de la Guerra y Noriega. |
| 1830s | The rise of the ranchos began, as vast areas of valley and coastal lands were given to applicants by the Mexican government. |
| 1836 | Mission San Buenaventura was secularized. By 1839, the mission was in decline. |
| 1847, January | John C. Fremont's party, heading to Los Angeles to participate in the Mexican War, passed through Ventura County, with little opposition. |
| 1848 | The Treaty of Guadalupe Hidalgo ceded California to the United States. |
| 1850, February 18 | The First California Constitutional Legislature divided the state into 27 counties, with the Ventura area being part of Santa Barbara County. |
| 1850, September 9 | California became a state of the United States. |
| 1860 | The ranchos were changing hands and broken up, as drought and depression resulted in a drop in cattle prices. |
| 1866, March 10 | The town of Ventura was incorporated, consisting of about one square mile around the mission. Angel G. Escandon, Walter S. Chaffee, Juan Camarillo, Victor Ustusaustegui, and Fernando Tico, Jr. were the town's first board of trustees. |
| 1868 | The coast stage line is established through Ventura County. |
| 1870 | A group led by W.E. Barnard settled in the coastal area of Rancho El Rio de Santa Clara o La Colonia, starting the town of Pt. Hueneme. |
| 1873 | Ventura County was organized and the city of Ventura was made the county seat. |
| 1887-8 | The Southern Pacific Railroad was extended to San Buenaventura, starting a real estate boom. |
| 1887 | Montalvo was established, organized by the Montalvo Land and Water Company. |
| 1887 | Piru was established, founded by David Cook. |
| 1887 | Bardsdale was founded by R.G. Surdam on land bought from Thomas R. Bard. |
| 1892 | The subdivision map for Somis was recorded. |
| 1897 | A beet sugar factory was built in the Oxnard area, establishing the sugar beet business in the county. |
| 1898 | Oxnard was surveyed and the town map was subsequently recorded by the subdivider, Colonia Improvement Company. |
| 1925 | The Ventura Avenue oil field north of the city of the city of Ventura came into production. |
| 1928, March 13 | The St. Francis Dam collapsed, sending a wall of water through the Santa Clara Valley to the sea. Three hundred and eighty-five people were killed. |
| 1946 | The Naval Air Missile Test Center at Point Mugu was established. |