In July 1987, he was General Director of the European Court of Human Rights, an affiliate of the Compagnie financière de Suez.

[5][6] However, the group soon went back to its original name, Suez and in 2003 its leaders made a proposition to shareholders to change the association of the company by becoming an LP with a board of directors and Gerard Mestrallet as the CEO.

[8] On 3 May 2016, Mestrallet left his position due to statutory age limits[9] and instead became chairman of the board of directors of the group, with Isabelle Kocher appointed as CEO, as planned for months.

[11] Also in 2014, Mestrallet's retirement cap of 21 million euros was a controversy in the media due to GDF Suez's record losses for 2013 and its effort to prepare a savings plan.

[13] In 2016, Mestrallet was appointed by Minister of Ecology Ségolène Royal to chair a three-member advisory committee to review European carbon pricing and make recommendations to the French government.

[14] In 2018, Mestrallet was appointed by President Emmanuel Macron as Executive Chairman of the French Agency for the Development of Al-ʿUla in Saudi Arabia, a French organization in charge of tourism and cultural development in the area in cooperation with the Saudi Kingdom.