Following his return from Siberia, Sejima joined C. Itoh & Co. in 1958, where he was initially engaged in its aircraft trading business. He became a director in 1962, just four years after joining the company. Sejima led the corporate planning team at C. Itoh, where he implemented military-style reporting methods and formed a group of followers within the company known as the "Sejima machine." He took charge of Itoh's expansion into the oil industry and arranged an alliance between General Motors and Isuzu in 1971. He was also one of three key men involved in Itoh's entry to the People's Republic of China in 1972, making it one of the first Japanese companies to do business with the country. Sejima was also instrumental in Itoh's merger with Ataka & Co. He was promoted to deputy president in 1972, deputy chairman in 1977 and chairman in 1978; he left the chairmanship in 1981 but remained an executive advisor to the company until 2000. During the 1980s, he served as a member of the Ad Hoc Commission on Administrative Reform and as an advisor to Prime Minister Yasuhiro Nakasone. In this capacity, he aided council chairman Toshio Doko in the privatization of NTT and the Japanese National Railways. Sejima went on to advise Prime Ministers Keizo Obuchi, Kiichi Miyazawa and Ryutaro Hashimoto. He was a director of NTT from 1986 to 1999. In 1998, he was appointed to head a panel examining reform of the Ministry of Finance. He was also chairman of the board of Asia University and the Chidorigafuchi National Cemetery. Sejima also developed a close relationship with the military rulers of South Korea during the 1980s. Samsung founder Lee Byung-chul invited Sejima to Korea in 1980 in order to advise Chun Doo-hwan and Roh Tae-woo "as a fellow man of the military." Sejima acted as a go-between for Nakasone in arranging an historic meeting with Chun in 1983. A committee led by Sejima erected a monument to Indian judge Radhabinod Pal at Tokyo's Yasukuni Shrine in 1997. Pal had been the dissenting judge in the guilty verdict handed down by the Tokyo War Crimes Tribunal.
In media
Works by Sejima
Works related to Sejima
The novel Fumō Chitai, which has been adapted as a film and as two television series, is said to be based on the life of Sejima, although the author stated that she only borrowed the main character's progression from Sejima's life story. Sejima's role in Japan-Korea relations during the 1980s was depicted in the Korean television series 5th Republic.