Sébastien was previously head of the Insurance Markets and Products Unit (2009-2012) and of the Corporate Financing Unit at the Directorate-General of the Treasury (2012-2013), where he also served as General Secretary of the Interministerial Committee for Industrial Restructuring. He was appointed deputy assistant secretary in 2013 (until 2018), in charge of Financial Markets and Corporate Financing.

He graduated in 1997 from Ecole Polytechnique.