Illinois 48 runs southwest to northeast from Interstate 55 to east of Clinton. It is an undivided surface street for its entire length. It also serves the cities of Raymond, Harvel, Taylorville and Decatur.
SBI Route 48 ran from Onarga to Raymond via the current Illinois 48 and Illinois 54. The portion from DeWitt to Onarga became U.S. Route 54 in 1942, but in 1972 U.S. 54 was pulled back to its current terminus in Pittsfield, and Illinois 54 was implemented from Springfield to Onarga.