Concord Active Property Listings Concord is the largest city in Contra Costa County, California, USA. At the 2010 census, the city had a population of 122,067.Area: 31 sq miles (81 km²)Population: 122,067 (2010) Concord Active Listings Concord – City Website Share this:EmailTwitterLinkedInFacebookDiggStumbleUponRedditPrint