Counties are geographical and political subdivisions of the state. They serve as administrative units for state and some federal laws, programs and services. Principal county functions include general government, protection of persons and property, health and sanitation, roads and bridges, recreation, welfare and corrections, and land use planning.

California's Constitution of 1879 required the Legislature to establish a uniform structure of government for counties. All counties were then governed by established general law, hence the term "general law county." The Constitution was later amended to permit any county to adopt a charter determining the structure of its government, hence the term "charter county." As a general law (rather than a charter) county, Marin County must elect supervisors by district and must elect, not appoint, certain officers. The supervisorial districts must be roughly equal in population, not area, and their boundaries must be adjusted every ten years following the federal census.

The Board of Supervisors is the legislative and executive body of county government. Supervisors pass all ordinances governing the county and are responsible for seeing that mandated functions are properly discharged. They adopt the budget, set employee salaries, direct public works, including the county road system, and serve ex-officio on the boards of county service districts and regional agencies concerned with traffic, land use, air quality, transportation, among many others. Supervisors oversee a great variety of services to all county residents, including voter registration, health and welfare programs, courts, jail facilities, and the recording of official documents. They also must provide such municipal type services as fire and police, planning, zoning, land use regulation, street maintenance, traffic regulation, parks and recreation in the unincorporated areas of the county.

The Board of Supervisors appoints a County Administrator who is responsible for implementing board decisions, preparing the county budget, providing the supervisors with the information they need for making decisions, and coordinating the administration of county government. Their jurisdiction covers only county areas, not the incorporated cities and towns.

Also elected are the court judges, the Board of Education, the Superintendent of Schools, and eight county officials. Other officials, department heads, are appointed by the Board of Supervisors. Their duties and powers are prescribed by law. Compensation of judges is fixed by the State Legislature. Salaries of all other officers are fixed by the Board of Supervisors.