[1] Illinois 99 runs northwest from Meredosia to Mount Sterling.

It enters the Mount Sterling area on Pittsfield Road north of Illinois Route 107.

It then overlaps U.S. Route 24 and runs west on Main Street.

After about a half mile (.8 km), Illinois 99 then turns north on Cross Street, west on Washington Street, and then back north on Camden Road en route to Camden.

Illinois 99 continues north (staying west of the LaMoine River) to Illinois 101 at Brooklyn, its northern termination.