- Martinez is the county seat of Contra Costa County, California, United States. The population was 35,866 at the 2000 census. The downtown is notable for its large number of preserved old buildings. Martinez is located on the south side of the Carquinez Strait in the San Francisco Bay Area, directly facing the city of Benicia. Martinez is noted for its good schools, including Morello Park and St. Catherine of Siena.Please click on the categories on the left side of the page to explore them!