The city of Los Angeles stayed within its initial 30 square-mile area until the 1890s. The first significant enlargements to the city were the districts of Highland Park and Garvanza to the north, and the South Los Angeles area. In 1906, the approval of the Port of Los Angeles and an adjustment in state law allowed the city to annex the Harbor Gateway, a thin strip of land leading from Los Angeles towards the port. San Pedro and Wilmington were incorporated in 1909, and the city of Hollywood was added in 1910, making the city 90 square miles. Also annexed that year was Colegrove and Cahuenga, as well as part of Los Feliz.

The opening of the Los Angeles Aqueduct gave the city with four times as much water as it needed, and the availability of water service became a powerful lure for nearby communities. Los Angeles administrators locked in clients through annexation by declining to supply other communities. By referendum of the residents, 170 square miles of the San Fernando Valley were annexed to the city in 1915, almost tripling its area. Over the next twenty years dozens of new additions brought the city's area to 450 square miles. Currently, it is approximately 470 square miles.

In the Second World War, Los Angeles grew as a center for production of war supplies and munitions. Thousands of African Americans and white Southerners migrated to the area to fill factory jobs.

By the middle of the century, L.A. was an industrial and financial colossus created by war production and migration. Los Angeles assembled more cars than any place other than Detroit, produced more tires than anywhere but Akron, and stitched more clothes than any city except New York. Additionally, it was the national center for the creation of films, radio programs and television shows. Building and construction boomed as tract houses were built in suburban communities financed by the Federal Housing Administration.

Los Angeles continued to spread out, particularly with the development of the San Fernando Valley and the construction of the freeway system in the 1940s. When the local streetcar line went out of business, L.A. became a locale entirely built around the motorcar.