Since 2009 Mathieu has been Associate Professor in American Civilisation at the Université Paris Sorbonne (Paris 4). He is also Adjunct Research Fellow at the Australian National University, where he helped to establish the Virtual Observatory for the Study of Online Networks in 2005. In 2008-2009 he worked at the Australian Department of Broadband, Communications and the Digital Economy on a project to map digital identity programs across public service agencies. Mathieu's book Cyberchiefs: Autonomy and Authority in Online Tribes was published by Pluto in 2009. In 2010 he founded and became the editor of the online journal Critical Studies in Peer Production and was elected to the Board and Bureau of Les Amis du Monde diplomatique.

His research addresses the impact of Information and Communication Technology on the production and reproduction of power within social groups. He is currently interested in how the emergence of new forms of organisation and expertise in the online environment can be related to the issue of class. He is also working on quantitative research methods into online organisational fields such as environmental and social movement activists.