Intimate partner violence (IPV) is experienced by one-third of women globally, yet few programs attempt to shift men’s IPV perpetration. Community mobilization is a potential strategy for reducing men’s IPV perpetration, but this has rarely been examined globally. We conducted a mixed-methods process evaluation alongside a trial testing community mobilization in peri-urban South Africa. We used in-depth interviews (n=114), participant observation (160 h), and monitoring and evaluation data to assess program delivery. Qualitative data (verbatim transcripts and observation notes) were managed in Dedoose using thematic coding and quantitative data were descriptively analyzed using Stata13. We learned that outreach elements of community mobilization were implemented with high fidelity, but that critical reflection and local advocacy were difficult to achieve. The context of a peri-urban settlement (characterized by poor infrastructure, migrancy, low education, social marginalization, and high levels of violence) severely limited intervention delivery, as did lack of institutional support for staff and activist volunteers. That community mobilization was poorly implemented may explain null trial findings; in the larger trial, the intervention failed to measurably reduce men’s IPV perpetration. Designing community mobilization for resource-constrained settings may require additional financial, infrastructural, organizational, or political support to effectively engage community members and reduce IPV.