[1] By the mid-1830s, Clark was in the employ of Isambard Kingdom Brunel as an engineer on the construction of the Great Western and Taff Vale Railways.

[1] Involvement in major earth-moving works seems to have fed his interest in geology and archaeology and he, anonymously, authored two guidebooks on the railway, in addition to a critique of Brunel's methods, which was published in Gentleman's Magazine in 1895.

[1] From 1843 to 1847, Clark worked on the Great Indian Peninsula Railway, surveying and planning the first passenger line in India, from Bombay to Thana which was opened in 1852.

In 1865, Clark purchased Tal-y-garn Manor, a small property near Llantrisant, Glamorgan, and set about building an estate of some 924 acres (3.74 km2) with the intention of founding a landed dynasty.

The works had been, for a while, in some decline and Clark took rapid steps to improve management controls, attracting additional capital and persuading Henry Austin Bruce to share with him the responsibility of the trusteeship.

The consequent expansion of the industry, and the difficulty of procuring an adequate supply of suitable ores at home, led Clark, in conjunction with the Consett Iron Company and Messrs. Krupp of Essen, to acquire an extensive tract of iron-ore deposits near Bilbao in Spain.

As his wealth grew, he delegated the day-to-day management to Menelaus, his trusteeship terminating in 1864 when ownership passed to Sir Ivor Guest.

Under his regime Dowlais became in effect a great training school which supplied to similar undertakings elsewhere a much larger number of managers and leading men than any other iron or steel works in the country.

[1] Clark published in six volumes Cartae et Alia Munimenta Quae ad Dominium de Glamorgancia Pertinent ("Charters and Other Muniments which Pertain to the Lordship of Glamorgan").

It consists of transcripts of some 1,660 ancient charters, numbered in Roman numerals, in their original language and spelling, which Clark had searched out from various sources including the muniments of Margam Abbey and Ewenny Priory.

Clark took little interest in party politics but was an opponent of protectionism and served on a royal commission on the coal trade (1866–1871).

[5] He was retained by the General Board of Health and worked on analysing the sanitary condition of towns and villages countrywide.