Early trials of the 'Marshal Ney' as a 15-inch-gun monitor were rather dismal, since it proved much slower and less manoeuvrable than expected, prompting the Admiralty to seriously consider scrapping it altogether and transferring its armament to a worthier ship. It was ultimately commissioned in 1916, but served only four months in that role before being converted into a permanent guardship. During April to June 1916 it was re-armed in Portsmouth Dockyard with a single 9.2-inch gun forward in place of the 15-inch gun turret and four 6-inch guns, two per side, on the upper deck. Wyllie's drawing shows her in this condition, with the box shields of the 6-in armament clearly visible. After limited service at Dover, during which the ship again showed her mechanical unreliability, she was returned to Portsmouth in August 1916 and paid off. Between January and March 1917 the 9.2-inch gun was removed and an additional pair of 6-inch guns were fitted so that she could be used as a permanent guardship. After her conversion she was sent to the Downs (off Deal) and took up her position at the north end of the Downs anchorage on 5 April 1917. She remained there until after the armistice. The 'Marshal Ney' was towed from her anchorage to Sheerness on 12 December 1918 and became a base ship for motor launches at Queenborough. She was completely disarmed in 1920 and spent the rest of her career as an accommodation ship before being broken up at Milford Haven in 1957.