This study explored longitudinal associations between personality and body-mass-index (BMI) in school-age children, including the potential mediating role of screen time and physical activity, and the potential moderating roles of child demographics and neighbourhood socioeconomic position. Participants were the parents (and teachers) of 3857 ten-year-old children, who completed questionnaires at baseline with a two-year follow-up. After controlling for child demographics (e.g., sex, pubertal status), we found that personality was unimportant for concurrent BMI, but was important for subsequent BMI and change in BMI over two years. Low levels of introversion and persistence at baseline, and decreases in persistence over time, were associated with a higher BMI at follow-up and a greater increase in BMI over time. Moderator analyses showed that introversion was more strongly related to subsequent BMI for children listed as aboriginal or Torres Strait Islander. The relationship between personality and change in BMI was mediated by screen time, but not by physical activity. To conclude, findings demonstrate that personality is important for change in body mass in Australian children (particularly indigenous children), and that screen-based sedentary behaviour features an important role in this association.