On 26 January 1939, Miller was commissioned as a second lieutenant into the Welsh Guards, and was given the service number of 85599. He served with the British Expeditionary Force (BEF) after the outbreak of the Second World War that September. Serving with the regiment's 1st Battalion, part of the 32nd Guards Brigade of Major General Allan Adair's Guards Armoured Division, he was a courageous soldier who distinguished himself during the war, and was honoured for his bravery during and after the Allied invasion of Normandy in June 1944. He was awarded the Military Cross (MC) in December 1944, while serving as a major commanding a company in the 1st Battalion, Welsh Guards, with the citation noting, "that in the North Western Europe theatre of operations, after D-Day, in the face of heavy shelling, he kept his beleaguered men together, continually exposing himself to enemy fire, with complete disregard for his survival."