Grundy County is a county located in the U.S. state of Iowa. At the 2020 census, the population was 12,329. The county seat is Grundy Center. The county is named for Felix Grundy, former U.S. Attorney General.

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.