Plas Cwmorthin was owned and built by the Rhosydd Slate Company Ltd in 1860 for their manager. Sheltered by a small group of conifers, it was an imposing rectangular building with symmetrical frontage, built of stone with large quoins, half-hipped roof and gable walls once slate-hung. Inside there were four rooms on the ground floor, with a cellar under one part. There was a walled garden to the front and on the east side, the latter still showing traces of cultivated beds. In the north-east corner was a two-hole privy.
The 1861 census records the agent's family with four children, two servants, a visitor and two lodgerrs who were clerks at Rhosydd. Successive managers occupied the house until the early 1930s, after which a family moved from the nearby Rhosydd Terrace (NPRN 408871).
W J Crompton, RCAHMW, 30 July 2009.