Grundy County is a county located in the U.S. state of Missouri. As of the 2020 census, the population was 9,808. Its county seat is Trenton. The county was organized January 2, 1841, from part of Livingston County, Missouri and named after U.S. Attorney General Felix Grundy.

Livingston County is a county located in the northwestern portion of the U.S. state of Missouri. As of the 2020 census, the population was 14,557. Its county seat is Chillicothe. The county was organized January 6, 1837, and named for U.S. Secretary of State Edward Livingston.