Assistant principals are important education personnel, both as essential members of school leadership teams and apprentice principals. However, empirical evidence on their career outcomes remains scarce. Using statewide administrative data from Tennessee and Missouri, we provide the first comprehensive analysis of AP mobility. While prior work focuses only on AP promotions into principal positions, we also account for APs who exit school leadership and transfer to a different school. We find yearly mobility rates of 25-28%, with 10% of APs leaving school leadership, 7.5% changing schools, and 7.5-10% 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. Principal transitions also appear to increase the likelihood that APs exit school leadership and change schools, highlighting an additional cost of high rates of principal churn.