San Mateo County is the only county in California that elects all its county supervisors at-large. The county’s charter revision group is considering whether to suggest a change to some other election system. The county has a population of 700,000, so running countywide is very expensive. See this story.
San Mateo County, like all but one county in California, has five county supervisors. San Mateo County does have five districts for county supervisor elections, but the only function of the districts is to require that one supervisor be a resident of each of the five districts. Residence aside, the districts have no function.
San Mateo County is the county that is south of San Francisco but north of San Jose.