Background: Communities of Practice (CoPs) are increasingly used in health and non-health sectors globally. Evidence suggests that CoPs can support health promotion activities, but the research mainly encompasses formal, professional contexts: the role and contribution of CoPs in community-centred health promotion has not been explored. This paper presents a process evaluation of a CoP that aimed to facilitate social innovation among voluntary, community, faith and social enterprise (VCFSE) organisations. Hosted by a city-region government in England, VCFSE organisations were invited to join a CoP to enable the development and implementation of their ideas for addressing gambling harms. Methods: The process evaluation sought to develop mid-level programme theory for the use of CoPs in community-centred health promotion. Data collection consisted of 33 qualitative interviews with stakeholders, as well as project reporting. Data were organised using a framework approach which supported the construction of themes and a complex intervention model. The research team reflected on these to develop the programme theory. Results: The CoP facilitated the development of community-centred interventions for addressing gambling harms through a two-track process: first, a community of VCFSE staff was formed, whose understanding of gambling harms was nurtured through discussions led by people with Lived Experience; second, the CoP contributed to project development via collaboration, knowledge sharing and an integrated referral pathway, although project-level benefits were uneven. Learning was generated in community engagement, training, education, support and social campaigns. Conclusions: The findings confirm the combined effectiveness of a CoP, varied VCFSE projects and people with Lived Experience to co-create an evolving knowledge-base for a city-region government’s gambling harms reduction strategy. CoPs may therefore complement partnership working in community settings, although additional training support may be required in comparison with CoPs involving health professionals. The city-region government’s approach could be replicated in other emerging public health areas.