Abstract: Under what conditions do UN peacekeeping operations (PKOs) carry out the tasks written into their mandates? Faithful mandate implementation is the backbone of peacekeeping legitimacy among its various stakeholders, including UN Security Council members, troop-contributing countries, host governments and citizens in conflict-torn countries. Yet, PKOs are increasingly tasked with implementing complex mandates. At the same time, PKOs are increasingly deployed to active conflict zones, as in Mali or South Sudan. We argue that as conflict dynamics shift quickly and unpredictably and complex mandates strain resources and invite coordination problems among specialized UN agencies, these two trends—increasingly complex mandates, increasingly implemented in active conflict zones—hinder the implementation of mandated peacebuilding and security tasks. We test this argument using new data on both peacekeeping activities on the ground and UN Security Council mandates in Africa from 1995 to 2016. Using different instrumental variable approaches and fixed effects models, we find that mandate complexity and government violence indeed reduce PKOs’ “process performance” in peacebuilding tasks. Our findings imply that the trend towards increasingly complex operations in ongoing conflict likely results in a mismatch between what PKOs are mandated to do and what they actually do on the ground.