Del Mar is a beach city in San Diego County, California, located on the coast of the Pacific Ocean. Established in 1885 as a seaside resort, t…Del Mar is a beach city in San Diego County, California, located on the coast of the Pacific Ocean. Established in 1885 as a seaside resort, the city incorporated in 1959. The population was 3,954 at the 2020 census, down from 4,161 at the 2010 census.