There are 7 US Post Offices in Putnam County serving a total of 6,113 residents equating to 873 residents per post office. It's estimated that approximately 9,757 packages are handled across the 7 county post offices annually. Putnam County is the least extensive county in the U.S. state of Illinois. According to the 2010 census, it had a population of 6,006. The county seat is Hennepin. The county was formed in 1825 out of Fulton County and named after Israel Putnam, who was a general in the American Revolution. Putnam County is part of the Ottawa-Peru, IL Micropolitan Statistical Area, which is also included in the Chicago-Naperville, IL-IN-WI Combined Statistical Area."