During the Napoleonic Wars, she was used as the flagship of the Admiral the Duke of Clarence (later King William IV). She took part in the bombardment of Algiers in 1816 under the command of Admiral David Milne where she was second in the order of battle. In the attack on Algiers, "Impregnable", isolated from the other ships was a large and tempting target, attracting attention from the Algerian gunners who raked her fore and aft, she was severely damaged. 268 shots hit the hull, the main mast was damaged in 15 places. "Impregnable" lost Mr. John Hawkins, midshipman, 37 seamen, 10 marines and 2 boys killed and Mr. G. N. Wesley, Mr. Henry Quinn, 111 seamen, 21 marines, 9 sappers and miners and 17 boys wounded. The "Impregnable" saw little further action, apart from a short commission in the Mediterranean, and in 1819 she was placed in the Reserve Fleet at Devonport. From May 1839 to October 1841 she had relieved as the Commander-in-Chief's flagship moored at the entrance to the Hamoaze. She then saw service again in the Mediterranean until May 1843, when she was once again laid up with the reserve fleet at Devonport.