There are 8 US Post Offices in Martin County serving a total of 20,466 residents equating to 2,558 residents per post office. It's estimated that approximately 32,666 packages are handled across the 8 county post offices annually. Martin County is a county located in the U.S. state of Minnesota. As of the 2010 census, the population was 20,840. Its county seat is Fairmont. Martin County was named for Henry Martin, a pioneer settler."