It runs just under fifteen miles (24 km) between the town of Castine and Orland at U.S. Route 1 (US 1) and SR 3/SR 15.

Battle Avenue carries the initial stretch before turning northward as Castine Road as the route exits the downtown area.

SR 166 turns to the northwest at this point to run closer to the Penobscot River along The Shore Road.

This portion of the road heads north through the settlement of West Castine passing some small houses along the way.

Continuing north, intersecting the northern end of the SR 166A half-loop at Morse Cove.

SR 166 curves to the west at Old Country Road and crosses the Orland River on a small bridge.

Now in the business district of the town, SR 166 passes small markets, boating shops, and a post office a peninsula known as Leaches Point.

The route originally ran along the west side of the peninsula on The Shore Road.

The town line between Castine and Penobscot begins to run down the road and does so for the remainder of its route.