BACKGROUND: An understanding of how to implement person-centred interventions in palliative and end of life care is lacking, particularly for supporting family carers. To address this gap, we investigated components related to successful implementation of the Carer Support Needs Assessment Tool (CSNAT) intervention, a person-centred process of carer assessment and support, using Promoting Action on Research Implementation in Health Services (PARIHS) as a theoretical framework. This study identifies how the PARIHS component of 'facilitation' and its interplay with the components of 'context' and 'evidence' affect implementation success. METHODS: MRC Framework Phase IV study to evaluate implementation of the CSNAT intervention at scale, over six months, in 36 UK palliative care services. 38 practitioners acting as internal facilitators in 35/36 services were interviewed. Field notes were collected during teleconference support sessions between the external and internal facilitators. RESULTS: Successful implementation was associated with internal facilitators' 'leverage' including their positioning within services, authority to change practice, and having a team of supportive co-facilitators. Effective facilitation processes included a collaborative approach, ongoing communication, and proactive problem solving to address implementation barriers. Facilitators needed to communicate the evidence and provide legitimacy for changing practice. Contextual constraints on facilitation included having to adjust recording systems to support implementation, organisational changes, a patient-focused culture and lack of managerial support. CONCLUSIONS: The CSNAT intervention requires attention to both facilitation processes and conducive organisational structures for successful implementation. These findings are likely to be applicable to any person-centred process of assessment and support within palliative care.