By 1 May, "Thunderer" had been assigned to the 3rd BS of the Home Fleet. On 1 November, the 3rd BS was disbanded and "Thunderer" was transferred to the Reserve Fleet at Portland, together with her sisters. She was temporarily recommissioned during the summers of 1920 to ferry troops to the Mediterranean and back. The ship was still in Portland as of 18 December 1920, but was converted into a naval cadet training ship at Rosyth beginning in February 1921. "Thunderer" recommissioned on 5 May, but did not begin her first training cruise until 24 June. The ship was relieved by the monitor on 31 August 1926 and paid off at Portsmouth.