Howlett was at Sandhurst during World War I on one of the shorter, war-time courses for young officers. He was commissioned as a second lieutenant in April 1918 into the Queen's Own Royal West Kent Regiment (RWK) and served in the Special Reserve in England throughout the summer of 1918. In August he was posted to the Western Front and served with the 7th Battalion of the RWK in the Second Battle of the Somme, seeing action almost immediately at Sailly-Saillisel. During the following months he fought in the more mobile advances around Ronssoy, Le Cateau and Cambrai until the Armistice in November.