An estimated 1.3 million stroke survivors living in the United Kingdom (UK) currently rely on family caregivers for daily support. The needs of stroke family caregivers are, however, not routinely assessed by most clinical services. Early identification of their needs and support is crucial to maintain their well-being and caregiver role. At present, stroke-specific caregiver screening tools are lacking. This mixed method, the multiphase study aimed to develop a Carers’ Alert Thermometer for stroke family caregivers (CAT-S) by adapting the CAT, a short screening tool developed in the context of end-of-life care. Underpinned by principles of action research, qualitative and quantitative data were collected sequentially between February 2016 to December 2017 from purposive samples of stroke family caregivers (n = 76) and staff working within stroke services (n = 238) in the UK. Semistructured interviews were conducted to inform the contents of the CAT-S. Key items for inclusion were identified through a modified Delphi survey and consultation with an expert panel. The CAT-S was then piloted in North West England to test its usability and usefulness in practice to identify the needs of stroke family caregivers. Thematic and content analysis were used to analyse qualitative data. Quantitative data were analysed using descriptive statistics. The CAT-S comprises the key challenges that are experienced by stroke family caregivers. Two additional items not present on the original CAT were identified and included; training needs of family caregivers to provide care and support for caregivers’ emotional needs. The CAT-S was found to be useful and acceptable by both staff and stroke family caregivers and resulted in action plans and support being provided. The CAT-S is a supportive tool for achieving person-centred care and prioritising stroke family caregivers requiring comprehensive assessments.