His father, Ryūtarō Sejima (瀬島龍太郎), was an officer in the Imperial Japanese Army who served under General Maresuke Nogi during the Russo-Japanese War, and was later the mayor of the village of Matsuzawa in Nishitonami District, Toyama.

He became a prisoner of war alongside General Otozō Yamada following Japan's surrender in 1945, and was detained for 11 years in Siberia.

[2] Following his return from Siberia, Sejima joined C. Itoh & Co. (now Itochu Corporation) in 1958, where he was initially engaged in its aircraft trading business.

[3] 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.

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.