A lawyer by profession, with a postgraduate diploma in law (DESS) in 1998, he began his career as a lawyer, then as a parliamentary attaché before working in a law firm. After a spell as head of legal affairs in a banking institution, he joined Bouygues Construction, where he held the position of Deputy General Counsel France, International and Africa for the Public Works subsidiaries.
Then, in 2017 he joined Colas, first as General Counsel of Colas Projects, then, in 2018, as General Counsel Major Projects & Financing of the Colas SA Group.
Guillaume joined Colas Rail in 2021 as General Counsel & Compliance Director and also became a member of the Executive Committee.