This study seeks to understand how, why and in what circumstances charity boards respond to multiple, external influences responsible for inconsistent organisational action. In complex settings where legitimacy is essential, organisations exposed to multiple and interconnected external stakeholder influences can become inconsistent in their actions. The Oxfam scandal and governance failures within UK charities that operate in Africa highlight this. Official reports point to the role of the board of trustees in monitoring the implementation of strategy. However, there is little guidance for boards. The literature on mission drift offers insight for understanding how inconsistent organisational action arises and persists. It suggests that organisations can resist, accept or accommodate external pressures that cause it. This evidence is limited; it mainly concerns donors and largely ignores the role of charity boards. The study is multi-method and provides a realist account. It includes collecting data from a survey of 95 charities, a literature synthesis of annual reports from charities and social sector literature about mission drift and interviews with 18 trustees and executives at four charities. The data is drawn from UK charities that operate in Africa. The African region reflects a complex setting - with multiple, interconnected external influences - where securing legitimacy is essential. The study finds that UK charity boards are somewhat involved in formulating strategy but less involved in monitoring strategy implementation. It identifies, tests and refines four mechanisms which define a suite of roles for the board in monitoring strategy and explains how these are impacted by decision-making contexts. The mechanisms and contexts are synthesised into a framework that describes how charity boards can manage stakeholder pressures to create an environment that is best for the charity. The study contributes to the literature on board involvement in strategy by arguing that mechanisms are the causes of board involvement. It advances models of mission drift and strategy formation by explaining how boards can go beyond resisting, accepting or accommodating external pressures, to influencing them. It makes a methodological contribution in applying realist approaches to strategy process perspectives and provides policymakers with evidence about how charities operate in complex environments. It suggests new governance practices to help boards manage sources of inconsistent organisational action.