Upon completion of training at the Royal Military College, Sandhurst, Jack Harman was commissioned into the 2nd Dragoon Guards (Queen's Bays) on 2 March 1940. Promoted to lieutenant on 2 September 1941, he served in the Second World War and was awarded the Military Cross in August 1943, "in recognition of gallant and distinguished services in the Middle East". He joined the 24th Lancers for the Normandy landings. In autumn 1944 he re-joined the Bay's for the Italian Campaign and was involved in the Spring 1945 offensive. During the Second World War, Harman served with the 2nd Armoured Brigade in the Western Desert and the Italian Campaign.