Shigemitsu was born in Ōita, Japan. He studied Law at Tokyo Imperial University, graduating in 1907. After World War I, He served briefly as consul at the Japanese consulate in Portland, Maine. On April 29, 1932, Shigemitsu was Japan's minister in Shanghai when a Korean independence activist, Yoon Bong-Gil threw a bomb at a reviewing stand Shigemitsu walked with an artificial leg and cane for the rest of his life.
Shigemitsu became ambassador to the Soviet Union, and in 1938 negotiated a settlement of the Russo-Japanese border clash at Changkufeng Hill. He then became Japan's ambassador to Great Britain until he was recalled home in June, 1941. He spent two weeks in Washington on the way back, conferring with Ambassador Nomura. Two days after Pearl Harbor, Shigemitsu was appointed ambassador to the Chinese puppet state in Nanking. On April 20, 1943, in a move that was viewed as a sign that Japan might be preparing for a collapse of the Axis, Japan's Premier Hideki Tojo fired foreign minister Masayuki Tuni in favor of Shigemitsu, who had been opposed to the militarists . The American press often referred to him in headlines as "Shiggy.
He, along with Yoshijiro Umezu, signed the instrument of surrender on September 2, 1945. He was later convicted of war crimes by taking part in Unit 731, and sentenced to 7 years imprisonment. He was paroled in 1950, and again served as Foreign minister from 1954 until 1956. He represented Japan at the UN General Assembly when it become the 80th member of the United Nations on December 18 1956. Prior to that, he was:
He died, aged 69, in Yugawara.