There are 15 US Post Offices in Winnebago County serving a total of 176,704 residents equating to 11,780 residents per post office. It's estimated that approximately 282,039 packages are handled across the 15 county post offices annually. Winnebago County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 295,266, making it the seventh most populous county in Illinois behind Cook County and its five surrounding collar counties. Its county seat is Rockford. Winnebago County is part of the Rockford, IL Metropolitan Statistical Area."