Marshall County is a county located in the U.S. state of Iowa. As of the 2020 census, the population was 40,105. The county seat and largest city 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, Iowa Micropolitan Statistical Area. In 2010, the center of population of Iowa was located in Marshall County, near Melbourne.