The first Europeans to visit Simí Valley were members of the Spanish Portolá expedition (1769–1770), the first European land entry and exploration of the present-day state of California.
The expedition traversed the valley on January 13–14, 1770, traveling from Conejo Valley to San Fernando Valley.
They camped near a native village in the valley on the 14th.

Rancho Simí, also known as Rancho San José de Nuestra Señora de Altagracia y Simí, was a  113,009-acre (457 km²) Spanish land grant in eastern Ventura and western Los Angeles counties granted in 1795 to Santiago Pico.
 After Santiago Pico's death in 1815, the Rancho was regranted to Santiago's sons Javier Pico and his two brothers, Patricio Pico and Miguel Pico.
Rancho Simí was the earliest Spanish colonial land grant within Ventura and Santa Barbara Counties.
The name derives from Shimiji, the name of the Chumash Native American village here before the Spanish.
It was the largest Spanish or Mexican land grant given in Ventura County, and one of the largest given in California.
The Simi Adobe-Strathearn House, later the home of Robert P. Strathearn and family, served as the headquarters of the rancho.

José de la Guerra y Noriega, a Captain of the Santa Barbara Presidio, who had begun to acquire large amounts of land in California to raise cattle, purchased Rancho Simí from the Pico family in 1842.
 After Jose de la Guerra's death in 1858, the sons of Jose de la Guerra continued to operate the ranchos.
 The end of their prosperity came when several years of drought in the 1860s caused heavy losses.
 In 1865, the De la Guerras lost the ownership of El Rancho Simí excluding the Rancho Tapo.
 El Rancho Tapo was part of the original 113,009 acre Rancho Simí grant, but sometime around 1820-1830, the Rancho Tapo came to be thought of as a separate place within Rancho Simí.
 The last of the De la Guerras to live in Simí Valley retreated to a 14,400-acre portion of the original rancho that was known as the Tapo Rancho.
 As late as February 1877, Juan De la Guerra was reported in county newspapers to be preparing to plant walnuts in the Tapo, which appears to be the final mention of their farming in relation to the original Simí grant.

The De la Guerra heirs tried every legal means, but by the 1880s, the Rancho Tapo also slipped from their ownership, as had the rest of the Rancho.
The first Europeans to visit Simí Valley were members of the Spanish Portolá expedition (1769–1770), the first European land entry and exploration of the present-day state of California. The expedition traversed the valley on January 13–14, 1770, traveling from Conejo Valley to San Fernando Valley. They camped near a native village in the valley on the 14th.
Rancho Simí, also known as Rancho San José de Nuestra Señora de Altagracia y Simí, was a  113,009-acre (457 km²) Spanish land grant in eastern Ventura and western Los Angeles counties granted in 1795 to Santiago Pico.  After Santiago Pico's death in 1815, the Rancho was regranted to Santiago's sons Javier Pico and his two brothers, Patricio Pico and Miguel Pico. Rancho Simí was the earliest Spanish colonial land grant within Ventura and Santa Barbara Counties. The name derives from Shimiji, the name of the Chumash Native American village here before the Spanish. It was the largest Spanish or Mexican land grant given in Ventura County, and one of the largest given in California. The Simi Adobe-Strathearn House, later the home of Robert P. Strathearn and family, served as the headquarters of the rancho.
José de la Guerra y Noriega, a Captain of the Santa Barbara Presidio, who had begun to acquire large amounts of land in California to raise cattle, purchased Rancho Simí from the Pico family in 1842.  After Jose de la Guerra's death in 1858, the sons of Jose de la Guerra continued to operate the ranchos.  The end of their prosperity came when several years of drought in the 1860s caused heavy losses.  In 1865, the De la Guerras lost the ownership of El Rancho Simí excluding the Rancho Tapo.  El Rancho Tapo was part of the original 113,009 acre Rancho Simí grant, but sometime around 1820-1830, the Rancho Tapo came to be thought of as a separate place within Rancho Simí.  The last of the De la Guerras to live in Simí Valley retreated to a 14,400-acre portion of the original rancho that was known as the Tapo Rancho.  As late as February 1877, Juan De la Guerra was reported in county newspapers to be preparing to plant walnuts in the Tapo, which appears to be the final mention of their farming in relation to the original Simí grant.
The De la Guerra heirs tried every legal means, but by the 1880s, the Rancho Tapo also slipped from their ownership, as had the rest of the Rancho.
Simi Valley, California