Williamson County is a county located in the southern part of the U.S. state of Illinois in the area known as Little Egypt. In the 19th century it was the site of a ten-year long feud known as the Bloody Vendetta that claimed many lives. According to the 2010 census, it has a population of 66,357, which is an increase of 8.3% from 61,296 in 2000. Its county seat is Marion. Williamson is a rapidly growing county in the Metro Lakeland area and is located southeast of St. Louis, MO. Its location at the intersection of Interstate 57, Interstate 24, and Illinois Route 13, a main east-west 4-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 .
Williamson County is often referred to as "Bloody Williamson" due to several outbreaks of violence that have few parallels in American history. These include the following: the Bloody Vendetta, 1876; the Carterville Massacre, 1899; Coal Strike, 1906; The Herrin Massacre, 1922; the Klan War, 1924–1926; the Birger/Shelton Gang War, 1926.
The Illinois National Guard was deployed repeatedly during the 1920s to separate the warring parties and attempt to keep order.
As with the entire United States, severe weather in Williamson County is not uncommon. The northwest section of the county was obliterated by the Tri-State Tornado of 1925. The county was also struck by 2 tornadoes on May 29, 1982 killing 10 people in the Marion, Illinois tornado outbreak. On May 8, 2009, the cities of Carterville, Herrin, and Marion were severely damaged by the May 2009 Southern Midwest derecho.