Shigemitsu was counselor of the Foreign Ministry from 1920 to 1924, chief secretary in the embassy in China from 1925 to 1927, and counsel general in Shanghai from 1927 to 1930. He became Japanese minister to China in 1931. During the Manchurian Incident, Shigemitsu sought direct negotiations between Japan and China rather than working through the League of Nations. In spite of his efforts to improve relations with China, the Japanese army escalated the crisis. In April 1932, a Korean nationalist tried to assassinate him with a hand grenade. Shigemitsu was badly wounded, losing a foot.
Shigemitsu served as deputy minister of foreign affairs from 1933 to 1936. He resigned because of the failure of his hard-line approach in negotiations with China. He was then successively ambassador to the Soviet Union (1936–1938), to Great Britain (1938–1941), and to the Wang Jingwei (Wang Ching-wei) government in China during 1942.
Shigemitsu became minister of foreign affairs in the government of Prime Minister Tojo Hideki in April 1943. In attempting to justify the war, Shigemitsu stressed that Japan sought to liberate Asian peoples from western oppression. At the Greater East Asian Conference held in November 1943, he sought to secure attendees' approval for Japanese policies. Shigemitsu continued as minister for foreign affairs and minister for Greater East Asia in the government of Prime Minister Koiso Kuniaki. He objected to the mediation plan to the Nationalist government of China advanced by Miao Bin (Miao Pin) because he doubted Miao's intention.
Shigemitsu resigned as minister of foreign affairs in April 1945 and was followed by Togo Shigenori. In August 1945, Shigemitsu returned as minister of foreign affairs in the government of Prime Minister Prince Higashikuni Naruhiko and signed the instrument of surrender on board the U.S. battleship Missouri on 2 September 1945. Arrested as a Class A war criminal, Shigemitsu was sentenced to seven years in Sugamo Prison, but he was released in 1950.
Shigemitsu again became minister of foreign affairs in the Hatoyama cabinet in December 1954, and he remained in office until December 1956. Among his accomplishments were the reestablishment of diplomatic relations with the Soviet Union and Japanese entry into the United Nations in 1956. Shigemitsu died in Yugawara, Kanagawa, on 26 January 1957. Sakai Kazuomi
Ito Takashi. "Shigemitsu Mamoru and the 1955 System." In Kataoka Tetsuya, ed., Creating Single-Party Democracy: Japan's Postwar Political System, 110–118. Stanford, CA: Hoover Institution Press, 1992.; Shigemitsu Mamoru. Japan and Her Destiny: My Struggle for Peace. F. S. G. Piggott, ed.; Oswald White, trans. New York: E. P. Dutton, 1958.