There are 13 US Post Offices in Columbia County serving a total of 56,850 residents equating to 4,373 residents per post office. It's estimated that approximately 90,739 packages are handled across the 13 county post offices annually. Columbia County is a county located in the U.S. state of Wisconsin. As of the 2010 census, the population was 56,833. Its county seat and largest city is Portage. The county was created in 1846. At the time, it was in the Wisconsin Territory. Columbia County is part of the Madison, WI Metropolitan Statistical Area as well as the Madison-Janesville-Beloit, WI Combined Statistical Area."