Meaningful 'user involvement' is an established aim of social work practice, and increasingly, an aspiration of criminal justice, yet there are unique challenges to participatory work within punitive contexts. Drawing upon a study of peer mentoring in the voluntary sector, this article unveils some core tensions related to (ex)service user involvement in criminal justice. Interviews with mentors, mentees, and key stakeholders, along with direct observations of practice, reveal that respondents often see their work as personal-political, emphasising the value of lived expertise and of collective action to address limiting social conditions. Simultaneously, however, mentoring is framed nationally and shaped locally by more established aims to correct, improve, and manage, individual 'offenders'. There is, therefore, a fundamental tension between processes of politicisation, or coming together to assert a user voice and affect social change; and professionalisation, wherein mentors are co-opted into forms of practice they often critique.