There are 9 US Post Offices in Fergus County serving a total of 11,581 residents equating to 1,287 residents per post office. It's estimated that approximately 18,485 packages are handled across the 9 county post offices annually. Fergus County is a county in the U.S. state of Montana. As of the 2010 census, the population was 11,586. Its county seat is Lewistown. The county was founded in 1885 and named for James Fergus, a Montana politician who was instrumental in creating the county (or, according to another source, John Fergus)."