In the 1920s, State Highway 109 followed today's US 44 between Routes 32 and 198 in eastern Connecticut. This highway was reconstructed in 1929 and served as an important segment of the route from Hartford to Providence via Putnam.
The modern Route 109, commissioned in 1932, ran between Bantam Lake (including what is now Route 209 and old Route 8 (now US 6). The portion of 109 between New Milford and Route 47 was called Route 134; the section between present-day Routes 47 and 209 wasn't numbered.
Route 134 was decommissioned in the 1940s. In 1963, that section became part of the extended and straightened Route 109; the north-south section along Bantam Lake, ending at what is now US 202, became Route 209.
In November 1967, a relocated section Route 109 opened near Black Rock State Park. The original alignment followed Old Branch Road, due about straight west before turning sharply north and meeting future Route 109 across from Old Northfield Road. The new Black Rock Dam and Reservoir, a flood control project undertaken by the U.S. Corps of Engineers, required moving Route 109.