There are 2 US Post Offices in Fallon County serving a total of 2,798 residents equating to 1,399 residents per post office. It's estimated that approximately 4,466 packages are handled across the 2 county post offices annually. Fallon County is a county located in the U.S. state of Montana. As of the 2010 census, the population was 2,890. Its county seat is Baker. The county was created in 1913 after being carved out of Custer County. The name comes from Benjamin O'Fallon, a nephew of Captain William Clark and an Indian agent for the upper Missouri region from 1823 to 1827."