The young Cubbon was educated at Alleyn's School and Dulwich College before attending and graduating from the Royal Military College, Sandhurst. He was posted to the Indian Army as a second lieutenant on 6 September 1911. His first assignment was to the York and Lancaster Regiment in Karachi. He subsequently was appointed to the 72nd Punjabis on the North-western Frontier of India, on 1 December 1912. He was promoted to lieutenant on 6 December 1913, and to captain on 6 September 1915. In November 1915, he was invalided home. Like so many invalided and convalescent land soldiers of the First World War, Cubbon volunteered for flight duty and was accepted as an observer on 25 March 1917. By Bloody April, 1917, he was assigned to 20 Squadron as an observer in Royal Aircraft Factory F.E.2s. Frederick Libby, the United States of America's first ace, gave a vivid description of an observer's duties aboard the aircraft that was an incremental development of the pre-1914 Farman Experimental:  And Libby did not even mention the hazards of spilling overboard with a propeller chopping along behind the crew. Cubbon scored two victories on 24 April 1917 with Lieutenant R. E. Johnson in F.E.2 number A6392. He then flew with Captain Frederick Thayre for the next six weeks and claimed some nineteen victories. Seventeen of these were over German Albatros D.III single-seated fighters. Upon Captain Albert Ball's death on 7 May, Cubbon became the second ranking ace of the Royal Flying Corps. On 9 June, two days after scoring their final victory together, Cubbon and Thayre attacked an Albatros two-seater and sent it down in a smoking nose dive. They were then killed in action by a direct hit from anti-aircraft fire from K Flak Battery 60 near Warneton. A German message drop confirmed their deaths to the British authorities, but their graves remain undiscovered. The nineteen victories shared included five Albatros D.IIIs shot down in flames and eleven destroyed, an Albatros reconnaissance two-seater in flames and another destroyed. Another D.III was claimed driven down 'out of control'. Cubbon also added his two victories with Johnson—a D.III destroyed and one 'out of control'. Cubbon received both the Military Cross on 11 May and a Bar in lieu of a second award on the 16th, both being gazetted posthumously on 18 July 1918.