The case study presented in this paper is focused on an effectiveness evaluation of a unique community-based intervention to improve childhood vaccination uptake in migrant populations residing in urban slum dwellings. This particular intervention was not only community-based, but it also relied on the community to co-deliver the intervention. Our paper discusses the methodological approach used for evaluating the effectiveness of this intervention along with the practicalities and challenges associated with the conduct of the evaluation. We used a program-theory-driven approach to guide the evaluation study design by mapping the causal pathway and underpinning assumptions between program activities and observed outcomes. This suggested the value of using a mixed-methods evaluation design including both quantitative and qualitative components. For the quantitative component, we used a multiple group post-test evaluation design (six intervention and six control communities) which is a quasi-experimental design. We used stratified random sampling of migrant population households to assess vaccination rates. We used multilevel logistic regression modeling for the analysis to account for possible clustering of effects as well as baseline vaccination rates. The findings showed that overall vaccination coverage significantly doubled in intervention communities as compared to control communities. The qualitative component provided insights into whether the key assumptions in our theory of change were met and if this observed increase in vaccination uptake could realistically be attributed to the intervention. We also examined baseline vaccination rates from previously conducted surveys in these communities to strengthen our causal inference.