Why do Islamists mobilize in some mosques, but not others? We answer this question by matching a list of mosque-based lectures, sermons, and collections carried out by the Egyptian Muslim Brotherhood in interwar Cairo with a geo-referenced 1:5,000 scale map series from the same time period. Our results suggest that the Muslim Brotherhood was more likely to operate in larger mosques where they enjoyed prior relationships and that were located in close proximity to transport networks. Mosques in areas lacking both government health services and other Islamic associations were also more likely to host Islamist activism. A supplementary analysis shows that the Muslim Brotherhood was more likely to establish health services in areas that had previously seen mosque activities. These findings deepen our understanding of the conditions under which certain mosques become sites for Islamist mobilization, and demonstrate how historical spatial data can be utilized to study political activism