There are 2 US Post Offices in Prairie County serving a total of 1,178 residents equating to 589 residents per post office. It's estimated that approximately 1,880 packages are handled across the 2 county post offices annually. Prairie County is a county located in the U.S. state of Montana. As of the 2010 census, the population was 1,179. Its county seat is Terry. Prairie County was created by the Montana Legislature in 1915 out of parts of Custer, Dawson, and Fallon Counties. The name was selected in a contest and reflects the predominant landscape of the region. The county was the site of the Custer Creek train wreck in 1938 that killed 47 people and injured an additional 75."