The road starts near downtown Pontiac and runs as the main highway north, intersecting with Interstate 55 and former U.S. Route 66.

Illinois 23 continues as a 2-lane highway from this point, turning west and running through Cornell.

Route 23 continues through Grand Ridge to Ottawa, where the road follows 1st Avenue and State Street, to form a concurrency with Illinois Route 71 across the Illinois River.

The highway once again splits into two one-way streets (Columbus and LaSalle Streets) through downtown Ottawa, until a block south of its junction with U.S. Route 6, where Illinois 71 departs toward the east.

North of Ottawa, Illinois 23 has a full diamond interchange with Interstate 80.