There are 3 US Post Offices in Pennington County serving a total of 14,214 residents equating to 4,738 residents per post office. It's estimated that approximately 22,687 packages are handled across the 3 county post offices annually. Pennington County is a county located in the U.S. state of Minnesota. As of the 2010 census, the population was 13,930. Its county seat is Thief River Falls. The county was formed on November 23, 1910, from sections of Red Lake County. It was named after Edmund Pennington, vice president of the Minneapolis, St. Paul and Sault Ste. Marie Railway."