There are 8 US Post Offices in Sheridan County serving a total of 3,270 residents equating to 409 residents per post office. It's estimated that approximately 5,219 packages are handled across the 8 county post offices annually. Sheridan County is a county located in the U.S. state of Montana. As of the 2010 census, the population was 3,384. Its county seat is Plentywood. Sheridan County was established in 1913 when the Montana Legislature split Dawson and Valley Counties. It was named after American Civil War General Philip Sheridan. Its northern boundary is the Canada\u2013United States border south of Saskatchewan."