Mental health can affect young people's sense of wellbeing and life satisfaction, their ability to participate in employment and education, and their onward opportunities in life. This paper offers a rare opportunity to longitudinally examine mental health in a population-representative study of children aged 4-5 years to 14-15 years. Using data from the Longitudinal Study of Australian Children (LSAC), this study examined maternally-reported child mental health over a 10 year period, in order to understand their initial mental health status early in life and its change over time, as measured by the Strengths and Difficulties Questionnaire. Longitudinal models were fitted from ages 4-5 to 14-15 years. Results showed that child sex, maternal mental health, socio-economic status (family income, maternal education, neighbourhood disadvantage), maternal hostility, and child temperament (persistence, sociability, reactivity) are all independent contributors to child mental health at age 4. These effects largely persist over time, with the effects of maternal mental health increasing slightly over time. Persistence of these effects suggests the need for early intervention and supports. The independent contribution of these factors to child mental health suggests that multi-faceted approaches to child and maternal mental health are needed.