18th Congressional District contains all or parts of 20 counties in Central and Western Illinois. A mixture of urban (Peoria, Springfield) and rural areas, the district contains rich, productive farmland and a strong manufacturing base. Counties in the 18th District include Adams, Brown, Bureau, Cass, Knox, Logan, Macon, Marshall, Mason, Menard, Morgan, Peoria, Pike, Putnam, Sangamon, Schuyler, Scott, Stark, Tazewell, and Woodford.

The 18th Congressional District contains all 11 of the counties that Abraham Lincoln represented during his one term (1847-1849) in Congress.