The first slate quarrying at Minllyn was from open pits before 1840. Quarrying for slate slabs continued underground with several headings but there was never a high extraction rate despite a standard gauge connection with the Mawddwy Railway at Dinas Mawddwy Station; a private branch from the Cambrian Railway at Cemmes Road. Although the line has a passenger service it was primarily intended for slate quarry traffic.
A water powered mill on the dressing floor alongside the open pit by 1845 and was the first integrated mill in the region employing 3 saws, 3 planers and slate dressing machines; the water wheel was later replaced by a pelton wheel with steam back up. From the mill there was a steep incline down to the valley below with a further short incline to the Mawddwy Railway.
The quarry eventually closed but was reopened and re-equipped in 1872 and for a short period a workforce of over 100 produced an annual extraction of 100 tons per annum. A new larger mill with 40 machines was built on the valley floor. By 1894 the workforce had been reduced to 20 with 550 tons of slate being produced.
Production continued to decline until the quarry closed in 1925, by this time there were only 3 saws and 2 planers. Tramways throughout the quarry were an unusual gauge of 2’ 4 1⁄4”; today much of the tramway still remains in place both on the surface and underground.
On the valley floor the later mill has been renovated as the Meirion Woolen Mill and craft centre. A number of ruinous buildings remain in the upper mill area; these include the mill, compressor house, workshops, weigh-houses and the drum house at the top of the main incline. A number of adits are accessed from part way down the incline but these do not connect with the main workings above. One of these adits leads to a chamber with the remains of a wooden bridge and an open air shaft close to the drum house on the upper dressing floor. An partially stone lined tramway tunnel runs from the upper dressing floor into the open pit. A rock fall at the mouth of the tunnel has caused a build up of water for the first few yards; a partly submerged mine waste truck stands on the track near the tunnel mouth. From the tunnel mouth a long abandoned incline serving the upper workings on the hillside can still be seen.
On the opposite side of the pit is the adit into the underground workings with random headings and chambers; all workings below adit level are now flooded. It is possible to climb out through an air shaft of one of the chambers. Some plant still remains in place including several winches, climbing chains and a boiler.