Social work is one of the core professions within mental health services in the UK, though its position as local authority ‘outsider’ excludes it from health-based workforce planning, leaving the exact national composition of mental health social work shrouded in mystery. In the context of increasing demand and limited resources, this lack of clarity precludes effective use of the mental health workforce. This study aimed to map the provision of mental health social work across England and Wales. A cross-sectional design was used, with brief surveys issued to all local authorities and National Health Service trusts responsible for providing social work or mental health services. These surveys collected the number of mental health social workers in each agency and their employment contexts for subsequent descriptive statistical analysis. Responses were received from 96.6 percent of agencies, employing 6,584 social workers. Findings indicated that mental health social work varies widely across both nations, with little clear rationale for, or consistency in, how social work is structured within mental health services. While these findings establish the first robust framework for understanding the size and scope of mental health social work provision, further research is required to establish temporal and contextual understandings to inform future service planning.