HMS Agamemnon and her sister HMS Lord Nelson were the last pre-dreadnoughts built for the Royal Navy. Both ships actually entered service after the revolutionary HMS Dreadnought, meaning they were already obsolete when completed. The completion of the two ships was delayed because the Royal Navy wanted HMS Dreadnought in service quickly, so the main armament turrets intended for Lord Nelson and Agamemnon were fitted to Dreadnought instead. As a result of having to have all new turrets built, HMS Agamemnon finally commissioned in June 1908.
By WW1 both ships were no longer part of the Grand Fleet as they were too slow compared to the newer 'Dreadnought' style battleships, so were assigned to the channel fleet. Both Lord Nelson and Agamemnon were transferred to the Mediterranean and took part in the Dardanelles campaign. During that campaign, Agamemnon was hit by shore based gunfire but none of it caused any really serious damage. Agamemnon remained in the Mediterranean for the rest of WW1 and returned to the UK in 1919, were she was placed in reserve. She was then converted into a radio controlled target ship before being scrapped in 1927