On 4 September 1923, "Mutsu" loaded supplies at Uchinoura Bay, Kyushu, for the victims of the Great Kantō earthquake. With her sister "Nagato", she sank the hulk of the obsolete battleship on 7 September 1924 during gunnery practice in Tokyo Bay, in accordance with the Washington Naval Treaty. Captain Mitsumasa Yonai, later Prime Minister of Japan, assumed command on 10 November. The ship was transferred to the reserve on 1 December 1925. "Mutsu" served as flagship of Emperor Hirohito during the 1927 naval manoeuvres and fleet review. Captain Zengo Yoshida relieved Captain Teikichi Hori on 10 December 1928. On 29 March 1929, the ship was assigned to Battleship Division 3, together with three light cruisers.