All of IL-123's route is contained within Sangamon and Menard counties.

Major towns located on or adjacent to IL-123 include Athens, Petersburg, and Williamsville.

New Salem, the home of Abraham Lincoln in the 1830s, has been reconstructed as Lincoln's New Salem State Historic Site near Petersburg on IL-123.

SBI Route 123 was what Illinois 123 is now, plus a road from Ashland south to Alexander at Interstate 72/U.S.

In October 2003, Illinois 123 was extended east to Williamsville, replacing some of Illinois Route 124 in the process as well as all of Sangamon County Route 11.