Assistant principals (APs) are important education personnel, but empirical evidence about their career outcomes remains scarce. Using administrative data from Tennessee and Missouri, we provide the first comprehensive analysis of AP mobility. While prior work focuses on promotions into principal positions, we also examine APs exiting school leadership and transferring across schools. We find yearly mobility rates of 25% to 28%, with 10% of APs leaving school leadership, 7.5% changing schools, and 7.5% to 10% becoming principals. We also document a strong relationship between AP mobility and principal turnover, where higher-performing APs are substantially more likely to replace their departing principal. Finally, principal transitions appear to increase the likelihood that APs exit school leadership and change schools.