BACKGROUND: The importance of supporting family carers is well recognised in healthcare policy. The Carer Support Needs Assessment Tool is an evidence-based, comprehensive measure of carer support needs to facilitate carer support in palliative home care. AIM: To examine practitioner perspectives of the role of the Carer Support Needs Assessment Tool intervention in palliative home care to identify its impact and mechanisms of action. DESIGN: Qualitative - practitioner accounts of implementation (interviews, focus groups, reflective audio diaries) plus researcher field notes. SETTING/PARTICIPANTS: A total of 29 staff members from two hospice home-care services - contrasting geographical locations, different service sizes and staff composition. A thematic analysis was conducted. RESULTS: Existing approaches to identification of carer needs were informal and unstructured. Practitioners expressed some concerns, pre-implementation, about negative impacts of the Carer Support Needs Assessment Tool on carers and expectations raised about support available. In contrast, post-implementation, the Carer Support Needs Assessment Tool provided positive impacts when used as part of a carer-led assessment and support process: it made support needs visible, legitimised support for carers and opened up different conversations with carers. The mechanisms of action that enabled the Carer Support Needs Assessment Tool to make a difference were creating space for the separate needs of carers, providing an opportunity for carers to express support needs and responding to carers' self-defined priorities. CONCLUSION: The Carer Support Needs Assessment Tool delivered benefits through a change in practice to an identifiable, separate assessment process for carers, facilitated by practitioners but carer-led. Used routinely with all carers, the Carer Support Needs Assessment Tool has the potential to normalise carer assessment and support, facilitate delivery of carer-identified support and enable effective targeting of resources.