Background Reducing health inequalities is a challenge for policymakers and civil society. A multisectoral and multilevel approach is most promising to reduce those inequalities. Previous research showed what key elements of Zwolle Healthy City, an integrated community-based approach aimed at reducing socioeconomic health inequalities, are. To fully understand approaches that are complex and context dependent, questions as 'how does the intervention work' and 'in what context does it work' are just as important as 'what works'. The current study aimed to identify mechanisms and contextual factors associated with the key elements of Zwolle Healthy City, using a realist evaluation perspective. Methods Transcripts of semi-structured interviews with a wide range of local professionals were used (n = 29). Following realist evaluation logic in the analysis of this primary data, context-mechanism- outcome configurations were identified and thereafter discussed with experts (n = 5). Results How mechanisms (M) in certain contexts (C) were of influence on the key elements (O) of the Zwolle Healthy City approach are described. For example, how, in the context of the responsible aldermen embracing the approach (C), regular meetings with the aldermen (M) increased support for the approach among involved professionals (O). Or, how, in the context of available financial resources (C), assigning a program manager (M) contributed positively to coordination and communication (O). All 36 context-mechanism-outcome configurations can be found in the repository. Conclusion This study showed what mechanisms and contextual factors are associated with the key elements of Zwolle Healthy City. By applying realist evaluation logic in the analysis of primary qualitative data we were able to disentangle the complexity of processes of this whole system approach and show this complexity in a structured manner. Also, by describing the context in which the Zwolle Healthy City approach is implemented, we contribute to the transferability of this ...