King joined the Royal Flying Corps in February, 1917. In the fall of 1917, he was assigned to No. 43 Squadron RFC to fly Sopwith Camels. In conjunction with fellow ace Lieutenant William MacLanachan, he drove down an Albatros D.III out of control on 12 November 1917. King did not score again until February 1918; in the latter half of the month, he ran off a string of three more enemy craft driven down out of control, then became an ace by destroying an Albatros D.V on the 26th. By March 1918, he had advanced to B Flight Commander. On 22 March 1918, he was awarded the Military Cross. He also scored three more times in March. He won the French Croix de Guerre in July 1918 for strafing and bombing enemy troops during the Second Battle of the Marne. On 3 August 1918, King added a Distinguished Flying Cross to his medals. By 8 August, his tally had grown to 19 victories, including eight straight in Camel no. D1864. He then switched to Sopwith Snipe no. E8031 for his final three victories. A summary of King's triumphs shows he singlehandedly destroyed eight enemy aircraft, shared two other destructions with other pilots, drove down seven enemy by himself, and cooperated with other pilots in driving down five others.
T./2nd Lt. Cecil Frederick King, Gen. List and R.F.C. For conspicuous gallantry and devotion to duty. On five occasions during a period of three months he has sent down four enemy machines completely out of control, and has destroyed one other. Later, under very adverse weather conditions he carried out a low reconnaissance, during which he engaged troops in a station, causing several casualties, fired into a body of the enemy entering a village from a height of 50 feet, attacked four gun limbers, causing the teams to stampede, and finally dived on to a parade of troops, who scattered in all directions. He has displayed exceptional daring and skill, which, combined with a splendid dash and initiative, have set a fine example to his squadron.
Distinguished Flying Cross
2nd Lt. Cecil Frederick King, M.C. He is a fine leader who at all times shows great gallantry and skill in manoeuvring; his energy and keenness have brought his flight to a high standard of efficiency. He frequently descends to low altitudes to obtain good results from bombing, and shooting, and on several occasions he has brought down enemy aeroplanes.