Upon commissioning, Iron Duke was assigned to the Home Fleet as the fleet flagship. After the outbreak of the First World War, the Royal Navy was reorganized; the Home Fleet and the Atlantic Fleet were combined to form the Grand Fleet; Iron Duke retained herposition as fleet flagship, under John Jellicoe. The ship took part in all of the major fleet actions, though only one of them ended in combatthe sortie on 31 May that resulted in the Battle of Jutland. At Jutland, Iron Duke was assigned to the 3rd Division of the4th Battle Squadron, and was stationed in the centre of the Britishline.
Following the end of the war, Iron Duke was transferred to theMediterraneanFleet. During 191920, the ship operated in the Black Sea in support of the White Russians during the Russian Civil War. The Washington Naval Treaty of 1922 mandated that Iron Duke be removed from the active roster. However, she remained with the fleet for a short time, having been transferred to the Atlantic Fleet in 1926. This duty assignment lasted 3 years, after which she was removed from active service. She was demilitarised, to be used as a training ship. Two of her gun turrets and a good deal of her armour were removed, and her speed was reduced to 18 kn (33 km/h; 21 mph) through the removal of some of her boilers. She was later used as a depot ship in Scapa Flow, starting in 1939. After the outbreak of the Second World War that year, her remaining guns were removed to be used in shore defences. The Luftwaffe attempted to bomb her on 17 October 1939; several near misses caused significant damage. After repairs were effected, the ship resumed her duties as a depot vessel until the end of the war. She was finally sold for scrapping in 1946.