Acton, Montana. Acton is an unincorporated community in northwestern Yellowstone County, Montana, United States. It has a postal ZIP code (59002) and lies along Montana Highway 3 northwest of the city of Billings, the county seat of Yellowstone County. Its elevation is 3,802 feet (1,159 m), and it is located at coordinates |45|55|50|N|108|40|51|W|.