William Henry Clark served an apprenticeship with Head, Wrightson and Company and then worked in London for a time for the consulting engineer Sir Alexander Rendell. He visited the United States twice to inspect bridgework being manufactured there for the Gokteik Viaduct in Burma. In 1902, at the end of the Anglo-Boer War (1899-1902), he came to South Africa to take up an appointment as chief draughtsman to the Imperial Military Railways, which soon became the Central South African Railways. Following the establishment of the Union of South Africa in 1910 he was appointed as assistant bridge engineer of the South African Railways and Harbours and subsequently promoted to bridge engineer. He became a South African citizen.
Clark was responsible for the design and construction of a number of important railway bridges in South Africa, including those across the Buffalo River and Gouritz River, and the Blaauwkrantz (now Bloukrans) Bridge near Grahamstown. Later in his career he became resident engineer in charge of the design and construction of port and inland grain elevators, a post he held until his retirement in 1930. After retiring from the railways he was involved in the construction of the Beit Bridge over the Limpopo River and the steelworks of the South African Iron and Steel Industrial Corporation, Limited (Iscor) at Pretoria.
Clark became a member of the South African Institution of Mechanical Engineers in 1908. He was married to Edwina Jane Clark, born Maule, with whom he had four sons.