After having completed his scientific baccalaureate, Guillaume obtained his degree in telecommunications engineering as well as a Master from HEC, "Programme Grande Ecole", obtained in Paris in 1999. He started his career as a web entrepreneur, founding a digital company before selling it to Salesforce. He then joined SFR, where he successively held the positions of Director Chairman Office, Chief Financial Officer and Chief Marketing Officer. In 2015, he joined the Canal+ Group as Chief Marketing Officer.
In August 2017, Guillaume joined the Executive Committee of Proximus as Chief Consumer Market Officer. Thanks to his wide range of experience in marketing, finance and strategy in both the media and telecom sectors in France, he has a very deep knowledge and understanding of the new business models and market dynamics of these sectors. Since 1 December 2019, Guillaume is appointed as Chief Executive Officer of the Proximus Group and chairs the Proximus Executive Committee.
In addition, Guillaume is Chairman of the Boards of Directors of BICS and Telesign, as well as a member of the Board of Directors of Proximus Art.