BACKGROUND: Following the 2019 NHS Long-Term Plan, link workers (LWs) have been employed across primary care in England to deliver social prescribing (SP). AIM: To understand and explain how the LW role is being implemented in primary care in England. DESIGN AND SETTING: Realist evaluation undertaken in England. METHOD: Focused ethnographies around seven LWs from different parts of England. As part of this, we interviewed 61 patients and 93 professionals from healthcare and the voluntary, community and social enterprise (VCSE) sector. We reinterviewed 41 patients, seven LWs and a LW manager 9-12 months after their first interview. RESULTS: We developed four concepts around how LWs are integrated (or not) within primary care: Centralising or diffusing power; Forging an identity in general practice; Demonstrating effect; Building a facilitative infrastructure. These concepts informed the development of a programme theory around a continuum of integration of LWs into primary care - from being 'bolted on' to existing provision, without much consideration, to 'fitting in', shaping what is delivered to be accommodating, through to 'belonging', whereby they are accepted as a legitimate source of support, making a valued contribution to patients' broader well-being. CONCLUSION: SP was introduced into primary care to promote greater attention to the full range of factors affecting patients' health and well-being, beyond biomedicine. For that to happen, our analysis highlights the need for a whole system approach to defining, delivering and maintaining this new part of practice.