First Contact Physiotherapy Practitioners (FCPPs) are working across the UK to deliver musculoskeletal services to patients within general practice. Little is known about the impact of the model and how variation in delivery may lead to different experiences and outcomes of the service amongst patients and general practice staff. This study explored staff and patient experiences of First Contact Physiotherapy in general practice to determine what works, for whom, under what circumstances, and how. Qualitative interviews were conducted in general practice sites across Great Britain. Interviews were based on initial programme theories identified in an earlier realist review and consensus event. We conducted 80 remote interviews with practice staff and patients, and a further 3 interviews with respondents with other roles related to First Contact Physiotherapy provision. All interviews were analysed using a realist approach. Seven overarching theory areas were identified: 1. Awareness of FCPPs; 2. Communication and integration into practice; 3. FCPP approach in primary care; 4. FCPP additional qualifications; 5. Practice workload; 6. FCPP personal development; and 7. Employment models of FCPP. Three key areas were identified as integral to successful service delivery and implementation: FCPP integration; skillset and impact on resource use; and employment model. Findings from this study strengthen the evidence by providing a robust piece of empirical work about the key issues and contexts impacting successful implementation of the FCPP role into general practice, which will aid decision makers when developing First Contact Physiotherapy services.