Accessible Summary: What is known on the subject?: ‘Leave’ is a common occurrence for patients detained in mental health settings. The term covers multiple scenarios, for example short periods to get off the ward through to extended periods at home prior to discharge. Despite the frequency and importance of leave, there is very little research about how it is implemented and whether, and in what circumstances, it is effective. While there is legislation about leave in the Mental Health Act (1983) mental health services are free to implement their own policies or not to implement one at all. What the paper adds to existing knowledge?: The leave policies of NHS mental health services in England and Wales are highly inconsistent. The extent to which policies are consistent with guidance differs depending on which service is providing care. What are the implications for practice?: It is very likely that, because of inconsistencies between services and policies, practice also differs. Clinicians need to understand their responsibilities in the leave process to ensure that patients are supported in their recovery journey. Policymakers need to revisit leave policies in light of evidence from this study. Abstract: Introduction: Considerable guidance is available about the implementation of leave for detained patients, but individual mental health services are free to determine their own policies. Aim: To determine how consistent leave policies of NHS mental health services in England and Wales are with relevant guidance and legislation. Method: A national audit of NHS mental health services leave policies. Data were obtained through web searching and Freedom of Information requests. Policies were assessed against 65 criteria across four domains (administrative, Responsible Clinician, types of leave and nursing). Definitions of leave‐related terms were extracted and analysed. Results: Fifty‐seven (91.9%) policies were obtained. There were considerable inconsistencies in how policies were informed by relevant guidance: Domain‐level consistency was 72.3% (administrative), 64.0% (Responsible Clinician), 44.7% (types of leave) and 41.9% (nursing). Definitions varied widely and commonly differed from those in relevant guidance. Discussion: Mental health professionals are inconsistently supported by the policy in their leave‐related practice. This could potentially contribute to inconsistent practice and leave‐related patient outcomes. Implications for Practice: To ensure patients are treated fairly clinicians need to be aware of their responsibilities around leave. In some services, they will need to go beyond their organization's stated policy to ensure this occurs.