On April 29, 1932, while attending a celebration for the birthday of Emperor Hirohito in Shanghai, a Korean independence activist, Yoon Bong-Gil, threw a bomb at a reviewing stand killing General Yoshinori Shirakawa and wounding several others, including Shigemitsu.

Shigemitsu was highly critical of the foreign policies of Yōsuke Matsuoka, especially the Tripartite Pact, which he warned would further strengthen anti-Japanese sentiment in the United States.

Shigemitsu spent two weeks in Washington, DC, on the way back from Britain and conferred with Ambassador Kichisaburō Nomura to attempt to arrange for direct face-to-face negotiations between Japanese Prime Minister Fumimaro Konoe and US President Franklin D. Roosevelt.

Shigemitsu's many attempts to stave off World War II angered the militarists in Tokyo, and only two days after the attack on Pearl Harbor, he was sidelined with an appointment as ambassador to the Japanese-sponsored Reorganized National Government of China.

[6] He then again served as Minister of Foreign Affairs briefly in August 1945 in the Prince Higashikuni Naruhiko administration right before Japan's surrender.

Shigemitsu, as civilian plenipotentiary, along with General Yoshijirō Umezu, signed the Japanese Instrument of Surrender on board the battleship USS Missouri on September 2, 1945.

However, the tribunal was extremely lenient on the grounds that Shigemitsu had regularly opposed Japanese militarism and protested the POWs' inhumane treatment.

In January 1957, a year after his visit to the Soviet Union, Shigemitsu died of myocardial infarction at 69 in his summer home in Yugawara, Kanagawa.