Hollis was commissioned into the Royal Marine Light Infantry in 1914 and served in the First World War in the Grand Fleet and the Harwich Force. Between the wars he served on the staff of the Commander-in-Chief Africa Station and of the Plans Division at the Admiralty before being appointed Assistant Secretary of the Committee of Imperial Defence in 1936. He served in the Second World War as Senior Assistant Secretary in the War Cabinet Office. He was present at virtually every major decision during that period, attending all the major conferences—Washington, Cairo, Tehran, Yalta and Potsdam—and was instrumental in establishing what became known as the Cabinet War Rooms (now known the Churchill War Rooms).