Pentraeth Corn Mill lies in a narrow valley, thought to be a glacial overflow channel, at a point where the Afon Nodwydd reached the valley side from the NNW. The two-storey building, built of roughly-coursed rubblestone and 9.4 by 7.05 metres in plan , was derelict and roofless in 1975. The wheel pit, on the SSW gable, could have housed an overshot wheel 4.2 metres in diameter and 1.3 metres wide; the wooden octagonal axle lay part-buried in the bottom of the pit. Inside, the pit wheel pit 2.4 metres wide, and sockets for two bridge trees and parts of the hurst frame were visible. The wooden pulley and jackshaft for the sack hoist lay in an adjacent shed, and fragments of kiln tile indicated the former presence of grain drying facilities.
By 1986 the building had been converted to a dwelling, with an extension to the west-north-west elevation. Part of a monolithic millstone formed the lintel of a small window.
W J Crompton, RCAHMW, 25 June 2008.