There are 10 US Post Offices in Williamson County serving a total of 62,214 residents equating to 6,221 residents per post office. It's estimated that approximately 99,300 packages are handled across the 10 county post offices annually. Williamson County is a county located in the southern part of the U.S. state of Illinois. According to the 2010 census, it had a population of 66,357. Its county seat is Marion. Williamson County is included in the Carbondale-Marion, IL Metropolitan Statistical Area. It is located in an area known as Little Egypt. Williamson is a rapidly growing county in the Metro Lakeland area and is located 88 miles (142 km) southeast of St. Louis, Missouri. Its location at the intersection of Interstates 57 and 24, and Illinois Route 13, a primary east-west four-lane expressway, gives it access to the major communities of Murphysboro, Carbondale, Carterville, Herrin, Marion and Harrisburg. Although the Williamson County population is only 66,357, it is located in the heart of the Metro Lakeland area of Jackson-Williamson Counties where 120,000 citizens of Illinois make their home. Carbondale (14 miles west), Herrin and Marion are the key urban areas in Metro Lakeland, with a combined population of over 65,000. Over 235,000 people live within 35 miles (56 km)."