Mathieu is the CEO and co-founder of More in Common, a non-profit group established in 2017 working to unite divided societies. More in Common has quickly grown into a widely recognized organizations in our field with a team of 50+ people working in 7 countries (US, UK, France, Germany, Poland, Spain and Brazil). More in Common partners with 160+ organizations in politics, civil society, media, the film industry, business and philanthropy. More in Common has conducted some of the most widely quoted research on polarization and the landscape of public attitudes in the United States and Europe. The work of More in Common has appeared in hundreds of media articles including the front pages of The New York Times and Le Monde as well as Fox News, The Economist, The New Yorker, The Atlantic, MSNBC, The FT, the BBC, Le Figaro, Die Zeit, La Repubblica and many more.

Make.org is a leading European civic tech company with a mission to reactivate democracy before it is too late. Make.org regularly engages hundreds of thousands of citizens in online consultations on themes ranging from healthcare to the environment, working in partnership with government agencies, global companies, leading non-profits and the media.

Founding Executive Director of the New Cities Foundation, a highly successful non-profit he helped launch in 2010. The Foundation's vision is a world where cities drive global progress. Its mission is to shape a better urban future for all by fostering urban innovation and entrepreneurship. Working with a high-level Board of Trustees, Mathieu and his team raised over $10M in funding and launched successful projects in Europe, Asia, North America, Latin America and the Middle East. Mathieu led pilot projects on e-health with the City of Rio, GE and Cisco and on connected commuting with Ericsson and Waze in the city of San Jose, California. He played a key role in the design of the Foundation’s world-class events including the New Cities Summit and Cities on the Move (with Google), successful research fellowship programs and the Foundation's Global Urban Innovators program for startups.

From 2007 to 2010, Mathieu was an integral part of the UN political mediation team in Lebanon, Syria and the broader Middle East, based in Beirut. He served as Special Assistant to the international judge investigating the assassination of former Lebanese Prime Minister Rafik Hariri. From 2005 to 2007, Mathieu was a senior political officer working in Afghanistan, working in some of the most volatile areas in the country. He conducted numerous field mediations with local tribes, armed groups including the Taliban and the military (US Special Forces, NATO, Afghan military), leading a team of 20 people in southern Afghanistan.