Mill Valley, city (1990 pop. 13,038), Marin co., W Calif., a suburb on Richardson Bay, an inlet of San Francisco Bay; inc. 1900. It is a residential community set in heavily timbered hills and valleys; redwood trees predominate. Golden Gate Baptist Theological Seminary and an artists' colony are there. Mt. Tamalpais State Park is adjacent to the city, and Muir Woods National Monument (see National Parks and Monuments, table) is nearby.