Mid-century brought the greatest change in the recorded history of the valley. The hills
enclosing the Doon Valley contained abundant supplies of coal and ironstone. The
principal landowner, Colonel McAdam Cathcart arranged with the Houldsworths, who were proprietors of the Coltness Iron Works at Newmains, Lanarkshire to set-up an
Ironworks at Waterside three miles north of Dalmellington.

Soon the Doon Valley was alive with new industry, with blast furnaces, the iron company's railway system, and an influx of several hundred "strangers" settling into the new villages created by "The Company" at Waterside, Benquhat, Lethanhill and Craigmark. These "strangers" came mainly from Ireland to set-up and run the  furnaces, mines, workshops and railway and also to manage the whole enterprise.

Soon horse-drawn wagons were carrying loads of pig iron along the turnpike road to Ayr. The opening of the ironworks prompted the extension for the Glasgow & South Western Railway from Ayr to Dalmellington. The railway also assisted the development of the
tourist trade and Dalmellington became a popular base for fishing and walking holidays.

In 1917 work began on creating "School of Aerial Gunnery" at Loch Doon. The army spent a fortune on the installations, building roads, hangers, slipways even a cinema. An aerodrome was constructed beside Bogton Loch and Camlarg House was requisitioned as the Royal Flying Corps HQ. German POW's also worked here. The scheme was
abandoned after at least one million pounds had been spent to little purpose.

During WW2 the Doon Valley was a training ground for the army and various units were stationed here from time to time. After the war the old mining villages were closed and the families re-housed in new housing schemes in Dalmellington, Bellsbank and Patna.

By 1950 there were some 3,000 mineworkers in the Valley. Virtually every family depended on the industry in one way or another. Within thirty years these mines closed one by one. Chalmerston closed in 1959, Houldsworth in 1965, Beoch in 1968, and Minnivey in 1975.

In July 1978 the miners at Pennyvenie No7, which was the last deep mine in the area, worked their last shift, and an industry that had provided employment for five generations died.