There are 17 US Post Offices in Williamson County serving a total of 329,505 residents equating to 19,383 residents per post office. It's estimated that approximately 525,927 packages are handled across the 17 county post offices annually. Williamson County (sometimes abbreviated as \"Wilco\") is a county in the U.S. state of Texas. As of the 2016 census estimate, the population was 545,412. Its county seat is Georgetown. The county is named for Robert McAlpin Williamson (1804?\u20131859), a community leader and veteran of the Battle of San Jacinto. Williamson County is part of the Austin-Round Rock, TX Metropolitan Statistical Area. It was included with Austin in the Best Cities to Live in for 2009 by the Milken Institute It is located on both the Edwards Plateau to the west, consisting of rocky terrain and hills, and Texas Blackland Prairies in the east consisting of rich, fertile farming land. The two areas are roughly bisected by Interstate 35."