Cuthbert Clive Finlay was born at Stroud, New South Wales during 1894 to Doctor Sinclair and Ethel Finlay. He attended Sydney Grammar School and was a medical student at the University of Sydney at the outbreak of the First World War. Finlay was active in the University of Sydney Scouts and had reached the rank of sergeant.

He enlisted in the Australian Imperial Force on 3 March 1915 and was appointed acting corporal in 'C' Company, 17th Battalion. He was promoted to corporal 29 March 1915 and departed Australia with the 17th Battalion aboard HMAT Themistocles on 12 May 1915. Finlay, and the 17th Battalion, arrived at Gallipoli on 20 August 1915 where they remained until the evacuation in December.

Finlay was stationed at Quinn's Post and was promoted to second lieutenant in November 1915. After further training in Egypt, the battalion landed in France on 22 March 1916 and were engaged in the battle of Pozières between 25 July and 5 August 1916. Finlay received a gunshot wound to the arm on 27 July and was evacuated to the 27th Stationary Hospital. He returned to his unit on 12 August 1916 before being transferred to the 5th Training Battalion in November 1916. During April 1917, he was transferred again to a newly raised 61st Battalion.
During April 1917, he attended the Bombing School and left with qualifications as an instructor in bomb throwing. Finlay was promoted to the rank of captain on 22 June 1917 and returned to the 17th Battalion in November 1917.

Finlay returned to Australia aboard HMAT Ceramic and arrived in Sydney on14 March 1919. He was was discharged on 23 May 1919. Cuthbert Finlay returned to the University of Sydney to complete his medical studies and graduated with a Bachelor of Medicine in 1923 followed by a Master of Surgery in 1924.