It was formed in 1928 because a stretch of road between the Wisconsin state line at Richmond and Crystal Lake, Illinois, was incomplete.

It ran from Big Foot Prairie to Crystal Lake which roughly followed present-day US 14.

While US 12 moved north along then-Illinois Route 60 from Richmond to Des Plaines, US 12 was rerouted to bypass the Chicago Loop via US 45 and 95th Street.

This diversion subsequently redesignated the former route east of Des Plaines to City US 12, albeit along a slightly different path.

City US 12 on the South Side instead traveled along South Parkway (now Martin Luther King Drive), Midway Plaisance, and Stony Island Avenue before ending at US 12/US 20 (95th Street).