San Dimas is a city in the San Gabriel Valley of Los Angeles County, California, United States. As of the 2010 census, the city had a total population of 33,371. The city historically took its name from San Dimas Canyon in the San Gabriel Mountains above the northern section of present-day San Dimas. San Dimas derives its name from the Spanish name for Saint Dismas.
San Dimas is bordered by the San Gabriel Mountains range to the north, Glendora and Covina to the west, La Verne to its north and east side, Pomona to its south and east side, and Walnut to the southwest.
The first known European exploration of the area was in 1774, when Juan Bautista De Anza passed through on the first overland expedition of Las Californias, from New Spain-Mexico towards Monterey Bay. The area was originally developed in 1837 with the Mexican land grant from Governor Juan Bautista Alvarado to Ygnacio Palomares and Ricardo Vejar for the Rancho San Jose, then in Alta California. It later became known as La Cienega Mud Springs, so named because of local mud springs that created a riparian marsh and healing place. Palomares and Vejar conducted sheep and cattle operations on Rancho San Jose, also growing crops for consumption by the residents of the rancho. In the early 1860s, a severe drought decimated the ranch's population of sheep and cattle. Ygnacio Palomares died in 1864, and his widow began selling the ranch land in 1865.