IL 126 begins at an intersection with IL 47 in Yorkville, and travels as a rural two-lane road until it intersects historic Lincoln Highway and enters Plainfield as West Lockport Road.

For most of its length, IL 126 is an undivided two-lane surface road, until a high-speed ramp is used to access northbound I-55 at its eastern terminus.

The original State Bond Issue Route 126 (SBI 126) opened in 1929 to carry the then-new US 66 from Springfield to Litchfield, bypassing the numerous towns along IL 4.

[8] This resulted in the section of the highway from Plainfield to I-55/US 66 having had four route numbers in its history: 66, 126, 66 (again), and 126 (again).

Until US 30 was rerouted in 2008, IL 126 was concurrent with it for a short distance approaching Plainfield from the west, between Wallin Drive (part of Lincoln Highway) and Main Street where IL 126 travels northeast.