Governments around the world use internal control agents and agencies to ensure that public funds are well spent and to avoid corruption. Yet we know little about the origins and effectiveness of internal control units, particularly in subnational governments of the global south. This paper analyzes the case of Brazilian municipalities’ internal control systems, using administrative data and quasi-experimental designs to examine the drivers and consequences of control. Using cross-sectional data, I document that there remains significant variation in the resources, mandates, and actions of internal control systems. Second, I use panel data and a causal event study design to show that legal reforms of internal control systems lead to an increase in hires of internal controllers. Third, I show these legal reforms have downstream consequences in the control of patronage, leading to a reduction in the size of the municipal workforce, and a higher incidence of civil service contracts. Fourth, I show that randomized federal anti-corruption audits have significant effects on municipal control systems. Together, these findings highlight the relevance of local internal control institutions and call for further research on the causes and consequences of their strength.