The objective of this study is to examine the trajectory of internalizing problems across middle childhood among a population sample of Australian children, and to understand the timing of explanatory factors related to children’s development of internalizing problems, by using multiple-indicator latent growth curve modeling. Participants were children, parents, and teachers in the Longitudinal Study of Australian Children (LSAC) Kindergarten (K) cohort (n = 3,153). Mothers reported on children’s internalizing problems at 3 time points (6–7, 8–9, and 10–11 years). Explanatory factors included gender, emotional regulation skills, externalizing problems, peer relationships, parenting behaviors, socioeconomic status, and maternal mental health reported by mothers and teachers at 2 time points (4–5 years and 6–7 years). Growth modeling identified an increasing trajectory of internalizing problems over time. Initial levels were predicted by concurrent (6–7 years) emotional dysregulation, externalizing problems, angry parenting, and maternal mental health problems, as well as earlier (4–5 years) peer problems and maternal mental health. Escalation in internalizing problems was predicted by externalizing problems, peer problems, maternal mental health at 4–5 years, and emotional dysregulation and peer problems at 6–7 years. Girls had both higher initial levels and faster escalation of internalizing problems than boys. The findings provide ecological and developmental evidence and insights for effective intervention. Identifying and addressing early problems with peers may be particularly important to avoid the risk of escalating internalizing problems. Professional development sessions for educators to promote and support children’s emotional regulation and peer interaction skills are likely to have a positive impact on children’s well-being.