Illinois Route 121 runs northwest to southeast in central Illinois, from Lincoln, through Decatur and Mattoon, to Greenup.
The northwestern extent of Illinois 121 is concurrent with Illinois 10, starting at that route's interchange with Interstate 55 in Lincoln:
The northwestern extent of Illinois 121 is concurrent with Illinois 10, starting at that route's interchange with Interstate 55 in Lincoln:
After passing through Lincoln, Illinois 10 turns to head east as Illinois 121 moves in a southeasterly direction from here:
At Mount Pulaski, Illinois 121 passes under Illinois 54, but they are joined by a connector...
Illinois 121 then approaches Decatur from the northwest, meeting a folded diamond interchange with Interstate 72:
Illinois 121 joins Illinois 48, which comes from the south, to form a concurrency running east along the north side of Decatur:
Illinois 121 then turns southerly away from Illinois 48, to run along the east side of Decatur:
After two miles, Illinois 105 joins Illinois 121 south for one block:
And after this single block, Illinois 121 turns onto US Route 36 east at a 4-way intersection, as Illinois 105 continues south:
Illinois 121 escapes from Decatur heading east on US Route 36, then splits off of US Route 36 to head south to Mount Zion:
As Illinois 121 exits Mount Zion, it meets the northern terminus of Illinois 128 in Dalton City:
As Illinois 121 approaches Sullivan heading mostly eastward, Illinois 32 joins from the north...
In Sullivan, Illinois 121 deviates from Illinois 32, which turns southerly again...
On the north side of Mattoon, Illinois 121 joins US Route 45 south:
Then Illinois 16 joins both routes from the west in Mattoon:
After one more block, Illinois 121 and US Route 45 both turn away from Illinois 16 to head due south:
Illinois 121 and US Route 45 concurrently run through a diamond interchange with Interstate 57 south of Mattoon...
About eight miles south of Mattoon, Illinois 121 splits from US Route 45, which turns southwesterly...
After passing through Toledo, Illinois 121 ends at Illinois 130 in Greenup, at a point sandwiched between US Route 40 and Interstate 70.
