Thomas Mathiesen’s theories, his activism and his scholarship are needed, important and useful for contemporary researchers concerned with social change and social justice. At this moment in time, when strong social movements and abolitionism are back on the agenda, Mathiesen’s theories can not only be revitalised, but moved forward in the everlasting unfinished fashion. Mathiesen’s work might be of support to researchers and activists confronted with the ‘system members’ as he calls them, to identify the structures of power one is up against and its strategies. His work provides a roadmap of how to handle repressive powers to reach the long-term goal of penal abolition. In this article, I will outline Mathiesen’s central theories of penal abolition as they are connected to political activism, then trace the role of the police in his abolitionism and, finally, I will argue for the relevance and development of Mathiesen’s abolitionist thinking in the contemporary and burgeoning field of transformative justice and police abolitionism. I aim to show the continuing relevance of Mathiesen’s theories for contemporary abolitionist movements and scholarship, and how they contribute to push these theories forward into new areas.