<p>Peer support is a prominent recovery-oriented innovation in mental healthcare, with implementation efforts to integrate peer support workers reported worldwide. However, its sustainment, including the factors that shape it, remains underexplored. Sustaining evidence-informed innovations and their associated benefits is a global policy priority and a complex challenge for health care systems. Although recent research has advanced theoretical frameworks and tools to support sustainability planning and assessment, there is a growing need for empirical studies that examine how sustainment unfolds in practice.&#xa0;Drawing on qualitative interviews, this study explores the sustainment of peer support two years post-implementation in a mental health service overseeing community housing for adults with mental health challenges in Canada. During 2022 and 2023, nineteen individuals including managers, service providers and peers, and housing proprietors involved in implementing and sustaining peer support were interviewed. Data collection and analysis were guided by the Consolidated Framework for Sustainability Constructs in Healthcare to explore determinants influencing sustainment.&#xa0;Sustainment was influenced by a constellation of factors spanning the design and delivery of peer support, organisational setting and processes, involved actors, available resources, and external contextual conditions. These factors exerted positive, constraining, or ambivalent influences on sustainment. Positive influences included staff belief in peer support values, funding availability, perceived effectiveness of peer support, peer workers’ expertise, integration of peer support innovation and peers into organisational policies and routine practices, organisational leadership, and participant satisfaction with being involved in peer support implementation. Key challenges included limited human resources, high staff workload, and union and administrative constraints, particularly those related to the lack of formal employment regulation of peer support workers. The interplay of additional contextual factors, such as provincial (regional) policy guidelines, training efforts, and organisational culture, highlights the context-dependent nature of sustaining peer support.&#xa0;This study advances understanding of peer support sustainment by demonstrating that it emerges from a dynamic interplay of influences across multiple and interrelated levels. By conceptualising sustainability determinants as context-dependent and evolving influences rather than fixed barriers or facilitators, the study offers a refined lens for examining peer support sustainment in practice. These findings have important implications for policy, research and practice in supporting the long-term integration of peer support workers in mental healthcare.</p>