How can patrimonial decision-making by village-level leaders be reduced? Debates on this topic have focused on the possibility of change via pressure from above (superordinate leaders) or below (citizens). We test whether horizontal pressure from civil society leaders can increase local accountability. We analyze an experimental intervention in Zimbabwe in which 270 villages were randomly assigned to receive training sessions for village heads, leveraging the fact that a civil society leader was trained alongside the village head in one arm of the intervention only. The inclusion of civil society leaders increased village heads’ knowledge of regulations, made decision making procedures more open and less biased, and raised citizens’ trust in their village heads. There is mixed evidence on whether it improved the resolution of local problems. A quantitative and qualitative analysis of the mechanisms through which civil society leaders had these effects suggests they applied direct social pressure on village heads.