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.
