PURPOSE: Shared decision-making (SDM) and the wider elements of intersecting professional and lay practices are seen as necessary components in the implementation of mental health interventions. A randomised controlled trial of a user- and carer-informed training package in the United Kingdom to enhance SDM in care planning in secondary mental health care settings showed no effect on patient-level outcomes. This paper reports on the parallel process evaluation to establish the influences on implementation at service user, carer, mental health professional and organisational levels. METHODS: A longitudinal, qualitative process evaluation incorporating 134 semi-structured interviews with 54 mental health service users, carers and professionals was conducted. Interviews were undertaken at baseline and repeated at 6 and 12 months post-intervention. Interviews were digitally audio-recorded, transcribed verbatim and analysed thematically. RESULTS: The process evaluation demonstrated that despite buy-in from those delivering care planning in mental health services, there was a failure of training to become embedded and normalised in local provision. This was due to a lack of organisational readiness to accept change combined with an underestimation and lack of investment in the amount and range of relational work required to successfully enact the intervention. CONCLUSIONS: Future aspirations of SDM enactment need to place the circumstances and everyday practices of stakeholders at the centre of implementation. Such studies should consider the historical and current context of health care relationships and include elements which seek to address these directly.