De Guingand was educated at Ampleforth College and Sandhurst. He joined the Middlesex Regiment in 1919, was seconded to King's African Rifles, 1926-1931 and was the Officer Commanding Troops, Nyasaland, 1930-1931 before returning to his regiment.
He attended staff college at Camberley and at the outbreak of the Second World War became Military Assistant to the Secretary of State for War, Director of Military Intelligence, Middle East, 1942 before serving as Montgomery's chief of staff. He was responsible for the running of Montgomery's armies on their journey from Egypt to the Rhine.
Montgomery appointed De Guingand as his chief of staff soon after his arrival in the desert to supersede Claude Auchinleck.
De Guingand was to prove indispensable to Montgomery, not only in battle, but also in relations with the Americans. De Guingand seems to have been blessed with considerable diplomatic skills, which proved useful when serving with Montgomery.