Port William Mill, High Street, Port William

Port William Mill is a former grain mill dating from the late 18th to early 19th century. The building features a rectangular main block that is two storeys high, with an attic and basement. It has two lower two-storey jambs on the east and west elevations to the south, creating a T-plan. There are also two single-storey wings on the west elevation, along with several additional structures.

The exterior is constructed of rubble, with the south gable of the main block limewashed. It includes squared granite rybats and quoins, and various window openings, some of which are blocked with brick.

On the west elevation facing High Street, there is a two-storey jamb, possibly a former kiln, on the right side. To the left of centre, there is a single-storey wing, and a piended single-storey wing that is further advanced to the left, featuring doorways on the west and south sides. A basement doorway is located to the right of centre, with openings at the centre for both floors and an additional opening on the left at the first floor.

The north elevation has an opening at the basement level and openings on both floors. The east elevation includes a two-storey jamb, identified as a drying kiln, on the left, with several openings and a brick lean-to addition on the right return. There are two doorways on the right side and two openings on both floors.

The south elevation features a partly-rendered rubble lean-to store attached to the gable and jamb on the left. The main block has granite coped skews, and there is a rubble gablehead stack with granite quoins and cope on the north side. A modern stack is positioned to the left on the west pitch, and there is a chimney vent on the roof of the east jamb. Most roofs are covered with corrugated sheet asbestos, while the two single-storey wings on the west have purple and grey slates with red sandstone ridges.

Inside, there is an overshot water-wheel located in the main block to the north, which dates from 1988.