He is often known as David Davies Llandinam (from the place of his birth, Llandinam in Montgomeryshire), in order to differentiate him from others of the same name. During his lifetime, he was also known by the nicknames, "Top Sawyer" and "Davies the Ocean". His father was a sawyer and lived on the south side of the Severn valley in Llandinam so that in winter the house was shaded from the sun because of the enclosing hills. Once he had started to amass his fortune, he built a grand country mansion of the north side of the river that would enjoy sunshine all year round. This mansion is Bron Eirion which is maintained in good order to this day in the ownership of Girlguiding Cymru. In order to gain access to the house, Davies built the first iron bridge in Montgomeryshire.
Davies was a coal magnate, largely responsible for the industrialisation of the Rhondda Valley. He founded the Parc and Maendy collieries, and constructed the docks at Barry. He was also responsible for the construction of railways across rural mid Wales including the very difficult crossing of Tregaron Bog in the construction of the line between Lampeter and Aberystwyth in 1866. It is reputed that he bought up the entire year's production of sheep fleeces in Cardiganshire to lay as a foundation for the railway line on the bog.
He served as Liberal Member of Parliament for Cardigan from 1874 until 1885 when the constituency was abolished. He then represented Cardiganshire from 1885 until 1886. In the 1886 general election he stood as a Liberal Unionist candidate, but was defeated by William Bowen Williams, the Liberal candidate, by 9 votes.
His grandson, another David Davies, continued Llandinam's philanthropic activities and was elevated to the peerage. Llandinam's two granddaughters, Gwendoline Davies and Margaret Davies, donated their substantial art collection to the National Museum of Wales.