Sykes was a junior officer in the 15th Hussars before becoming interested in military aviation. He was the first Officer Commanding the Military Wing of the Royal Flying Corps before the First World War and later served as the Flying Corps' Chief of Staff in France during the 1914 and 1915. Later in the war, he served in the Royal Naval Air Service in the Eastern Mediterranean before returning to Great Britain where he worked to organise the Machine Gun Corps and manpower planning. In late 1917 and early 1918, Sykes was the deputy to General Wilson on the Supreme War Council and from April 1918 to early 1919 he served as the second Chief of the Air Staff.