Shigemitsu was counselor to the Foreign Ministry during 1920–1924, chief secretary to the embassy in China during 1925–1927, and consul general in Shanghai during 1927–1930. In 1931 he became Japanese minister to China. Following the Manchurian Incident that same year, he sought direct talks with China rather than see the matter referred to the League of Nations. In April 1932 he was badly wounded (losing a foot) in an assassination attempt by a Korean nationalist.
As deputy foreign minister during 1933–1936, Shigemitsu devoted himself to Japan's relations with China. Resigning over the failure of his hard-line approach, he was subsequently Japanese ambassador to the Soviet Union (1936–1938), Great Britain (1938–1941), and the Wang Jingwei government in China during 1942. Shigemitsu opposed the Tripartite Military Pact that bound Japan to Germany and Italy.
Shigemitsu became foreign minister in the cabinet headed by Tōjō Hideki in 1943 and remained in that post in the subsequent Koiso Kuniaki and Higashikuni Naruhiko cabinets. As such, Shigemitsu signed the World War II capitulation agreement aboard the U.S. battleship Missouri on 2 September 1945. The next year, he was arrested and detained as a Class A war criminal. Sentenced to seven years in Sugamo Prison, he was released in 1950.
Shigemitsu was elected to the Japanese Diet in 1952. In 1954 he assumed the posts of both deputy prime minister and foreign minister in the Hatoyama Ichirō cabinet. In this post, Shigemitsu worked to restore diplomatic relations between Japan and the Soviet Union and to gain Japanese entry into the United Nations (UN), both of which occurred in 1956. Shigemitsu died in Yugawara in Kanagawa Prefecture on 26 January 1957.
Tomoyuki Takemoto and Spencer C. Tucker
Takeda, Tomoki. Shigemitsu Mamoru To Sengo Seiji [Mamoru Shigemitsu and Postwar Politics]. Tokyo: Yosikawa Kobunkan, 2002.; Watanabe, Yukio. Shigemitsu Mamoru: Shanghai Jiken Kara Kokuren Kamei Made [From the Shanghai Incident to UN Affiliation]. Tokyo: Chuo Koron Shinsha, 1996.