Contra Costa County California Cities/Towns - Alamo, Antioch, Bay Point, Bayview-Montalvin, Bethel Island, Blackhawk-Camino Tassajara, Brentwood, Byron, Canyon, Clayton, Clyde, Concord, Crockett, Danville, Diablo, Discovery Bay, East Richmond Heights, El Cerrito, El Sobrante, Hercules, Kensington, Knightsen, Lafayette, Martinez (County Seat), Moraga, Mountain View, Oakley, Orinda, Pacheco, Pinole, Pittsburg, Pleasant Hill, Port Costa, Richmond, Rodeo, Rollingwood, San Pablo, San Ramon, Tara Hills, Tessajara, Vine Hill, Waldon, Walnut Creek

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Your Ad Here Contra Costa County (Spanish for "opposite coast") is a county in the U.S. state of California. As of the 2000 census, it had a population of 1,024,319. The county seat is Martinez. History Contra Costa County was one of the original counties of California, created in 1850 at the time of statehood. The county was originally to be called Mt. Diablo County, but the name was changed prior to incorporation as a county. The county's Spanish language name means opposite coast, because of its location opposite San Francisco.