Under what conditions did the first Islamist movements organize? Which social and institutional contexts facilitated such mobilization? A sizable literature points to social and demographic changes, Western encroachment into Muslim societies, and the availability of state and economic infrastructure. To test these hypotheses, we match a listing of Muslim Brotherhood branches founded in interwar Egypt with contemporaneous census data on over 4,000 subdistricts. A multilevel analysis shows that Muslim Brotherhood branches were more likely in subdistricts connected to the railway and where literacy was higher. Branches were less likely in districts with large European populations, and where state administration was more extensive. Qualitative evidence also points to the railway as key to the movement’s propagation. These findings challenge the orthodoxy that contact between Muslims and the West spurred the growth of organized political Islam, and instead highlight the critical role of economic and state infrastructure in patterning the early contexts of Islamist activism.