There are 13 US Post Offices in Marshall County serving a total of 40,595 residents equating to 3,123 residents per post office. It's estimated that approximately 64,794 packages are handled across the 13 county post offices annually. Marshall County is a county located in the U.S. state of Iowa. As of the 2010 census, the population was 40,648. The county seat is Marshalltown. The county was formed on January 13, 1846 and named after John Marshall, Chief Justice of the United States Supreme Court. Marshall County comprises the Marshalltown, IA Micropolitan Statistical Area. In 2010, the center of population of Iowa was located in Marshall County, near Melbourne."