Mexico's implementation of mandatory teacher assessments in 2013 was part of a group of federal reforms aimed at enhancing the quality of state-run education. The reforms elicited strong opposition from key stakeholders. Building on the idea that policy capacities are the set of capabilities necessary to perform policy functions, we examine the effect of subnational government capacities on the pace of implementation of the mandatory teacher assessment in Mexico, a country with one of the most powerful teachers' unions in the world. After conducting statistical analyses based on panel data encompassing information on subnational government capacities from 2015 to 2017, we find that while subnational governments' human resources and fiscal capacities are associated with higher proportions of evaluated teachers, repressive capacities do not seem relevant for this purpose. Our research offers valuable lessons for policy makers in terms of recognizing adequate resource allocation and predicting the speed of policy implementation, even in contexts of significant opposition.