 In the 1932 state highway renumbering, Route 20 was established as a single route incorporating old Highways 133 and 343, with an additional extension east of the Connecticut River along old Highway 105 (modern Route 190) to Stafford Springs (ending at what was then Route 15).Route 20 crossed the Connecticut River on Bridge Street and overlapped US 5 into Thompsonville.The Barkhamsted Reservoir was filled in the 1940s causing Route 20 to be relocated to go around the north end of the reservoir.