Introduction: Social prescribing enables healthcare professionals to use voluntary and community sector resources to improve support for people with long-term conditions. It is widely promoted in the UK as a way to address complex health, psychological and social issues presented in primary care, yet there is insufficient evidence of effectiveness or value for money. This study aims to evaluate the impact and costs of a link-worker social prescribing intervention on the health and healthcare use of adults aged 40-74 with type 2 diabetes, living in a multi-ethnic area of high socioeconomic deprivation. Methods and analysis: Mixed-methods approach combining (1) quantitative quasi-experimental methods to evaluate the effects of social prescribing on health and healthcare use and cost-effectiveness analysis and (2) qualitative ethnographic methods to observe how patients engage with social prescribing. Quantitative data comprise Secondary Uses Service data and Quality Outcomes Framework data. The primary outcome is glycated haemoglobin, and secondary outcomes are secondary care use, systolic blood pressure, weight/body mass index, cholesterol and smoking status; these data will be analysed longitudinally over 3 years using four different control conditions to estimate a range of treatment effects. The ranges where the intervention is cost-effective will be identified from the perspective of the healthcare provider. Qualitative data comprise participant observation and interviews with purposively sampled service users, and focus groups with link-workers (intervention providers). Analysis will involve identification of themes and synthesising and theorising the data. Finally, a coding matrix will identify convergence and divergence among all study components. Ethics and dissemination: UK NHS Integrated Research Approval System Ethics approved the quantitative research (Reference no. 18/LO/0631). Durham University Research Ethics Committee approved the qualitative research. The authors will publish the findings in peer-reviewed journals and disseminate to practitioners, service users and commissioners via a number of channels including professional and patient networks, conferences and social media. Results will be disseminated via peer-reviewed journals.