Marshall County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 12,640. Its county seat is Lacon.Marshall County is part of the Peoria, IL Metropolitan Statistical Area.HistoryMarshall County was formed in 1839 out of Putnam County. It was named in honor of John Marshall, Chief Justice of the United States Supreme Court, who died in 1835.
