It is now widely accepted that context matters in evaluations of the health inequalities impact of community-based social initiatives. Systems thinking offers a lens for uncovering the dynamic relationship between such initiatives and their social contexts. However, there are very few examples that show how a systems approach can be applied in practice and what kinds of evidence are produced when this happens. In this paper, we use data from ethnographic fieldwork embedded within a multi-site mixed method evaluation to demonstrate how a systems approach can be applied in practice to evaluate the early stages of an area-based empowerment initiative – Big Local (funded by the Big Lottery Fund and delivered by Local Trust). Taking place in 150 different local areas in England and underpinned by an ethos of resident-led collective action, Big Local offers an illustration of the applicability of a systems approach to better understand the change processes that emerge as social initiatives embed and co-evolve within a series of local contexts. Findings reveal which parts of the social system are likely to be changed, by what mechanisms, and with what implications. They also raise some salient considerations for knowledge generation and methods development in public health evaluation, particularly for the evaluation of social initiatives where change does not necessarily happen in linear or predictable ways. We suggest future evaluations of such initiatives require the use of more flexible designs, encompassing qualitative approaches capable of capturing the complexity of relational systems processes, alongside more traditional quantitative methods.