We document a divergence in the duration of rule for monarchs in Western Europe and the Islamic world beginning in the medieval period. While leadership tenures in the two regions were similar in the 8th century, Christian kings became increasingly long lived compared to Muslim sultans. We argue that forms of executive constraint that emerged under feudal institutions in Western Europe were associated with increased political stability and find empirical support for this argument. While feudal institutions served as the basis for military recruitment by European monarchs, Muslim sultans relied on mamlukism—or the use of military slaves imported from non-Muslim lands. Dependence on mamluk armies limited the bargaining strength of local notables vis-à-vis the sultan, hindering the development of a productively adversarial relationship between ruler and local elites. We argue that Muslim societies’ reliance on mamluks, rather than local elites, as the basis for military leadership, may explain why the Glorious Revolution occurred in England, not Egypt.