About 3,000 years ago, Chumash Indians moved into the region and lived by hunting rabbits and other game, and gathering grains and acorns. On-going excavations, archaeological sites, and polychrome rock paintings in the area provide a glimpse into the social and economic complexity of the ancient Chumash world. In 1770, Captain Gaspar de Portola led a party of Spanish explorers and missionaries, traveling north on the route that became known as El Camino Real. The party camped near a Chumash village, believed to be the site of present-day Westlake Village. Father Juan Crespi, chaplain and diarist of the expedition, wrote: “We are on a plain of considerable extent and much beauty, forested on all parts by live oaks and oak trees, with much pasturage and water.”
When the Spanish finally did settle the area, they were given huge land grants, the largest of which was Rancho Simi, given to the Pico family. When Mexico won independence from Spain in 1821, California became Mexican territory, and a few more land grants were given. When California was admitted to the union in 1850, most of the land that later became Ventura County was divided among only 19 families. The picturesque future Westlake Village site among rising knolls, arroyos, barrancas and ancient oaks was recognized as the central part of two Mexican land grants: Rancho El Conejo and Rancho Las Virgenes. In 1881, the Russell brothers purchased a large portion of the land for cattle ranching. According to Patricia Allen, historian and family descendant, Andrew Russell beat the competition in buying the land by racing across 6,000 acres (24 km²) on a fifteen-minute trip in a buckboard and sealed the deal with a $20 gold piece. The price per acre was $2.50. The area continued to be known as the Russell Ranch although it was sold in 1925 to William Randolph Hearst and again in 1943 to Fred Albertson. The Russell family leased back part of the land to continue its successful cattle ranch operation while the Albertson Company used the vast area as a movie ranch.
In 1963, the American-Hawaiian Steamship Company bought the 12,000 acre (49 km²) ranch for $32 million and, in partnership with Prudential Insurance Company, commissioned the preparation of a master plan for a “city in the country.” Prominent architects, engineers, and land planners participated in designing the new community, a prominent example of planned 1960′s-style suburbanism.
The original tract was divided by the Los Angeles/Ventura county line. In 1968 and 1972, the Ventura County side, two portions of Westlake Village consisting of 8,544 acres (35 km2), were annexed into the city of Thousand Oaks. In 1981, the Los Angeles County portion (3,456 acres (13.99 km2) or roughly 1/3) of the Westlake Village master community was incorporated as the City of Westlake Village. California state law prevents a city from existing in two separate counties, so the areas in Ventura County remained part of Thousand Oaks. To this day, many residents of the Ventura County portions of Westlake do not realize that they are actually within the city limits of Thousand Oaks.