People leaving prison typically face numerous barriers to successful reintegration – a situation reflected by high reoffending rates in many countries throughout the world. To support people to overcome these barriers, criminal justice systems often provide some form of post‐custodial supervision. Based on a multisite ethnographic study, this article provides an insight into post‐custodial supervision in England and Wales. It introduces a novel theoretical framework for understanding the dynamics of supervisory relationships, which at its core distinguishes between legitimacy based on processes and legitimacy based on outcomes. While both sources of legitimacy are desirable, the former is particularly important because the latter is largely dependent upon it. In short, constructive relationships between licencees and supervisors fail to develop when processes are neglected, which, in turn, prevents probation and hostel workers from fulfilling their supervisory roles effectively.