A former magistrate, Guillaume joined DGM Conseil as a partner in March 2023. Recognised as an expert in legal communications, he was previously a partner at Vae Solis Communications, a firm he joined in 2012.

Guillaume has a rich ministerial experience. He was press and communications advisor in the offices of the Ministers of Justice (Pascal Clément, Rachida Dati and Michèle Alliot-Marie) and of the Minister of Foreign Affairs (Michèle Alliot-Marie). Between 2007 and 2010, he created and held the position of spokesperson for the Ministry of Justice and Freedoms.

In 2011, he was entrusted with a mission to improve judicial communication at the Ministry of Justice.

He began his career as an examining magistrate, presiding judge and assessor in Criminal court before being appointed deputy prosecutor in charge of relations with the press for the Paris public prosecutor, Jean-Claude Marin.