Roughly 25% of all elections worldwide experience some form of violence. While existing research has made significant inroads to understanding the causes of election violence, less research has been done on how effective proposed prevention mechanisms are. Existing qualitative case studies from Kenya and Liberia suggest that security sector reform (i.e., training and professionalization of the police) is among the most effective tools in preventing election violence (Claes 2016). Using original event data on election violence and data on police professionalization, staffing, and pay from 20 parliamentary elections between 1832 and 1914, we provide the first quantitative within-country and -constituency assessment on how effective security sector reform is at curbing election violence. We find that the establishment of professionalized police forces reduced pre-election violence, especially large-scale riots and disturbances. Moreover, once a constabulary has been formed more and better-paid police forces further reduce the number of recorded incidents.