<p>While family peer support workers are increasingly put forward as a means of enhancing family involvement in adult mental health care, research on their impact and implementation remains scarce. This study explores both the added value of, and the preconditions for, the successful implementation of family peer support workers in adult mental health care. A qualitative design was employed, consisting of semi-structured interviews with four family peer support workers, six hospital policymakers, and ten mental health professionals from two psychiatric hospitals in Flanders, Belgium, that recently employed family peer support workers. Participants perceived family peer support workers as contributing to improved family engagement and more family-sensitive care. However, successful implementation was found to depend on a set of interrelated macro-, meso-, and micro-level preconditions, including: (1) a co-created and well-considered policy vision and implementation framework; (2) a clear yet flexible role description and competency profile; (3) a conducive organisational culture and departmental climate, as well as supportive professional attitudes; and (4) adequate time and resources. These preconditions provide a foundation for the everyday legitimacy of family peer support workers, which is realised through relational trust and collaborative practice. At the same time, their implementation remains pioneering work, characterised by ongoing negotiation, with stigma still surrounding both family involvement and experiential knowledge.</p>