In the late 1920s through 1931, today's Route 203 was called state highway 222.

Portions of Hill Road (formerly Barber Road) in Harwinton were added to the state highway system in 1935 and 1936 as SR 722. At the time, Barber Road was fairly straight, moving south, until it crossed the Thomaston town line; after a couple curves it reached Twomile Bridge, on the east bank of the Naugatuck River, west of where Minor Road ends now. Across Twomile Bridge, the road ended at the original Route 8. To the north, the village of Fluteville; to the south, Thomaston.

Thomaston Dam, a ghost town, and ghost routes

The area near the Route 8 junction was changed significantly when the Thomaston Flood Control reservoir and dam were completed in 1960. The original Route 8 is closed to traffic, and Fluteville no longer exists. The south end of Hill Road was rerouted to the east and south, cutting through Plymouth before returning to Thomaston. Twomile Bridge is also gone. Most of these features are still visible in aerial photos.

The modern Route 222 was created from SR 722 and local roads in 1963. Its original southern terminus was the corner of North Main Street (Route 8 at the time) and Hill Road in Thomaston; its length was about 6.7 miles.

On Sept. 9, 1967, the Route 8 freeway opened in Thomaston, triggering some route changes. Route 222 was extended south along North Main Street (old Route 8) and along East Main Street (former US 6) to end at Route 8, intersecting the new freeway twice.