Grundy County is a county located in the U.S. state of Illinois. According to the 2010 census, it has a population of 50,063. 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 located in Grundy County, just northeast of the village of Mazon.