This study examined the professional development interventions received by classroom teachers transitioning to school administration roles in the DepEd Division of Quezon and the mediating role of transition readiness in leadership effectiveness. Using a quantitative descriptive correlational design with mediation analysis, data were gathered from 95 respondents. Results showed that professional development interventions were implemented to a great extent, while transition readiness was rated at a leadership ready level. Respondents were perceived as highly effective in performing leadership functions to school and to people. Correlation analyses revealed significant positive relationships among professional development, transition readiness, leadership effectiveness and their combinations. Mediation results indicated that transition readiness significantly mediated the relationship between professional development interventions and leadership effectiveness. Findings emphasize the importance of readiness-focused professional development and mentoring to support effective leadership transitions among classroom teachers entering school administrator roles within public education systems.