Critical junctures in history can cast long-run shadows on institutional development. The death of Prophet Muhammad in the 7th century ushered in a period of Islamic conquest. By 1100, conquest introduced several unique institutional innovations (e.g., political authority enforced with elite slave soldiers) leading to a “classical” Islamic equilibrium of centralized autocracy that has persisted to the modern period. To substantiate these claims, I demonstrate that countries with greater exposure to Islamic conquest are less democratic in the modern era. Furthermore, to trace the causal channel, I exploit a differences-in-difference research design to show that states exposed to Islamic conquest experienced a robust increase in state centralization during the conquest period. The institutional legacy of Islamic conquest helps explain the persistence of a “democratic deficit” in many Muslim-majority societies.