Grundy County is a county in the U.S. state of Illinois. According to the 2020 census, it has a population of 52,533. Its county seat is Morris. Grundy County is part of the Chicago-Naperville-Elgin, IL-IN-WI Metropolitan Statistical Area. In 2010, the center of population of Illinois was in Grundy County, just northeast of the village of Mazon. Illinois's state fossil, the unique and bizarre Tully Monster, was first found in Mazon Creek.