In many places in the UK, social work is integral to mental health service delivery. Significant role erosion, however, has left the profession unclear about where it fits within modern mental health services. The 2016 Social Work for Better Mental Health initiative outlines five key mental health roles and has been adopted into national policy in England to combat this uncertainty, but the influence of this has not been explored. This study aimed to develop an understanding of how mental health social workers perceive and explain their role. Semi-structured interviews were undertaken with seven social workers based within one English National Health Service mental health trust covering a large geographical area and their responses analysed using Ritchie, Spencer and O’Connor’s Framework thematic model. Findings indicated that social workers only superficially engaged with the aspirational policy roles, instead presenting their own framework for what makes mental health social work distinctive. This was constructed around the context and intentions of practice, rather than around proscribed tasks and responsibilities. This study has significance for individual social workers and for organisations providing and planning mental health services in the UK and beyond, given the influence that practitioner perceptions can have on how they undertake their roles.