In the 1920s, State Highway 171 followed today's Route 142 in East Haven.

The old Route 171, commissioned in 1932, ran from Route 14 (now Route 66) in East Hampton to Route 149 in East Haddam, using parts of Main Street, Colchester Avenue (today's Route 16), Waterhole Road and Trowbridge Road.

In 1933 or 1934, Route 16 was created, and Route 171 was shortened to Waterhole and Trowbridge Roads, between 16 and 149. In 1940 or 1941, Route 171 was deleted entirely, and the roads turned over to the towns.

The modern route 171 came into being in 1959, leading from Route 198 at Kenyonville to Route 12 in Putnam. This was formerly part of state route 91, which had to be renumbered to make way for Interstate 91. The stretch of today's Route 171 leading into Union became part of Route 198.

In 1963, Route 198 and Route 171 were reconfigured in Union, Eastford and Woodstock to their present-day locations.