The southern terminus of NE-12 was originally at New London, Connecticut.

It travelled along present-day Route 32 (along the west bank of the Thames River) from New London to Norwich, Connecticut.

North of Norwich, Route 12 is a minor road closely paralleling Interstate 395.

Route 12 goes through the following towns: Groton, Ledyard, Preston, Norwich, Lisbon, Griswold, Canterbury (0.14 miles only), Plainfield, Killingly, Putnam, and Thompson.

Media related to Connecticut Route 12 at Wikimedia Commons