Objective: Sedentary behaviour is an important contributor to the development of non-communicable diseases worldwide. Less is known about the relationship between screen-based sedentary behaviour and child mental health problems. This study explores cross-sectional and longitudinal associations between screen time (electronic gaming and television viewing) and psychosocial well-being in early and late childhood. Method: Two independent samples of Australian children were used to explore associations. Data were collected from the parents of 3956 young children (age 6) and 3862 older children (age 10) at baseline with a two year follow-up. Results: After controlling for demographic and socioeconomic factors (e.g., sex, household income), we found that screen time was negatively associated with prosocial behaviour, and positively associated with hyperactivity, peer problems and conduct problems in both samples. We also found that high screen time related to the development of emotional symptoms in young children, and to the development of hyperactivity and conduct problems in older children, over two years. Important moderators were household income, parental education level, and neighbourhood socioeconomic position. Conclusion: These findings indicate that screen-based sedentary behaviour is related to the development of psychosocial difficulties in early and late childhood.