He was the biologist on Sir Ernest Shackleton's Imperial Trans-Antarctic Expedition of 1914–1917, and served as the director of the Scottish Home Department Marine Laboratory, at Torry, Aberdeen.
In 1911 he attained a BSc and became Zoologist to the Scottish Oceanographical Laboratory, Edinburgh, a post he held until he was appointed naturalist to the Marine Biological Association in 1913.
[2] He had a reserved manner, not given to laughing or joking,[citation needed] but with a strong work ethic and a passion for biology.
On 9 August 1914 the Endurance departed Plymouth, carrying Shackleton and his crew on what was intended to be the first expedition to cross Antarctica from the Atlantic to the Pacific via the South Pole.
Clark was a hard worker, and, despite his dour manner, quickly won the respect of the crew with his willingness to volunteer for some of the more arduous or unpleasant jobs aboard ship, although he was the butt of several jokes.
When the ship became trapped in the ice he continued with his work, dissecting penguins and recording the changes in the plankton levels in sea.
Clark managed to produce a primitive alcoholic beverage from methylated spirit, sugar, water and ginger which became known as "Gut Rot 1916" and was drunk with a toast to "Wives and Sweethearts" on Saturdays.