In 1984, Mestrallet started working as a special advisor for Suez, known today just as Suez. He has been its Chairman and CEO since 2001. He was named Deputy Delegate General for Industrial Affairs in June 1986. In July 1987, he was General Director of the European Court of Human Rights, an affiliate of the Compagnie financière de Suez. In January 1991 he started as the Deputy Director General of the company, but a month later he became the Managing Director and Chairman of the Management Committee of the Société General de Belgique to contribute to its development.