Mill Valley is a city in Marin County, California, United States, located about 14 miles north of San Francisco via the Golden Gate Bridge and 52 miles from Napa Valley. The population was 14,231 at the 2020 census. Mill Valley is located on the western and northern shores of Richardson Bay, and the eastern slopes of Mount Tamalpais. Beyond the flat coastal area and marshlands, it occupies narrow wooded canyons, mostly of second-growth redwoods, on the southeastern slopes of Mount Tamalpais.