It connects Clinton center, through the village of Winthrop, to the town of Chester.

Route 145 begins at an intersection with US 1 in Clinton and heads northeast, into Westbrook.

In Westbrook, it turns north, intersecting with I-95 and passing by the Messerschmidt Pond Wildlife Area, before continuing into the town of Deep River.

In 1942 or 1943, its northern end was relocated and extended northward to Route 80.

In 1962, it was extended to its current northern terminus at Route 148 and the portion south of US 1 became town roads.