Grundy County is a county in the U.S. state of Illinois. According to the 2020 census, it has a population of 52,533. [1] Its county seat is Morris. [2] 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 ...