The 1st edition of the historic 25-inch O. S. maps, published in 1885, depicts Derwen Mill as a working corn mill, but the 2nd edition of 1902 depicts it as disused. It took water from a weir on the Nant Rhyd-y-moch stream and stored it in a mill pond immediately south of the mill. The water wheel was on the east side of the small building, which may be incorporated into the much larger dwelling now on site. The site of the mill pond is now a caravan park.
W J Crompton, RCAHMW, 13th June 2019.