Atherton is an incorporated town in San Mateo County, California, United States. Its population was 6,823 as of July 2023 estimates. The town'…Atherton is an incorporated town in San Mateo County, California, United States. Its population was 6,823 as of July 2023 estimates. The town's zoning regulations permit only one single-family home per acre in new subdivisions, though smaller lots exist from prior zoning laws.