Route 128 begins at an intersection with US 7 on the west bank of the Housatonic River within the town of Sharon.

Route 128 then continues east for 1.8 miles (2.9 km), passing by a state highway picnic area, then turns south.

About 0.6 miles (0.97 km) beyond the picnic area, Route 125 splits off to the south heading for Cornwall center.

Route 128 is a two-lane rural collector road and carries daily traffic volumes of up to 1,900.

In 1922, the portion of the old Goshen and Sharon Turnpike between West Cornwall and Cornwall Hollow, which had by now reverted to public control, was designated as a secondary state highway known as Highway 309.