Truancy is a widespread problem in upper secondary education as it adds significantly school dropout risk. However, the underlying mechanisms of truancy remain to be studied. This longitudinal study of 1,853 Finnish students examined how change in student engagement from primary (Grade 6) to lower secondary school (Grades 7 and 9) predicted truancy in upper secondary education, and whether cynicism (losing interest in school) mediated the relation between engagement and truancy. Growth curve models showed that high levels of engagement in primary school and increases in engagement over time predicted less truancy in upper secondary education. Cynicism mediated the effects of student engagement on truancy: high initial levels and increases in student engagement predicted less cynicism, which, in turn, related to lower levels of truancy. Findings underscored the importance of student engagement (both directly and indirectly through cynicism) in reducing truancy, and that these associations can carry over two critical school transitions.