Marshall is located on the east shore of Tomales Bay. It has a population of about 400 , although it has long claimed a population of 50 and touted this as the sum of the elevation (15 feet or 4.5 m above sea level) and the speed limit of 35 miles per hour (56 km/h) . It is located approximately 15 miles (24 km) south east of Bodega Bay, on State Route 1. Its ZIP code is 94940.
The town is named after four brothers called Marshall, who set up a dairying industry there in the 1850s. Starting in the 1870s, Marshall was a stop on the North Pacific Coast Railroad connecting Cazadero to the Sausalito ferry.
There is still some dairying in the area, but nowadays the town's major commerce is in oysters and clams, for which it is a center. It also acts as a center for tourists visiting Tomales Bay and the neighbouring Point Reyes Peninsula.