William Clark was 34 years old when he enlisted on 28 July, 1915. He was born in Perthshire, Scotland, but living in Melbourne and was a draper by trade. He appears to have had only a sister,who was still living in Scotland, because he nominates her as his next of kin, Mrs Anne Thomson, living at 4 Fairyhill Rd., Kilmanock, Scotland. He was Presbyterian by religion and began his military service on 11 August 1915 with the 2oth Castlemaine Depot Battalion, transferring to the Army Medical Corps at Ascot Vale on 25 October. On 6 March, he was transferred to Broadmeadows camp, attached to the Australian Medical Corps Special Reinforcements.

William Clark's unit embarked for Egypt on 7 March, 1915, landing at Serapium on 22 April. Here he was taken on strength with the 4th Field Ambulance unit, which left Alexandria on 1 June for France. On 22 June, Will Clark and the 4th Field Ambulance Unit left for the battlefields of the Western Front. For  thirteen months he served at the Front before being wounded in action on 6 May, 1917, with a bullet wound in one leg. he was sent first to the No. 6 General Hospital at Rouen, before returning to England for treatment at the Reading War Hospital.

From hospital, William Clark marched in to the Australian Army Medical Corps, No 2 Training Camp, Parkhouse, finally rejoining his unit in France on 31 December, 1917. He proceeded to France from Folkestone and finally returned to England post-war, marching out on 8 April 1919. He was returned to Australia on the Mahia on 4 June, serving on the nursing staff on the voyage home.

He was awarded a Victory Medal, British War Medal and 1915-16 Star for his service to his country.