There are few data from South Africa and other low- and middle-income countries on how mental disorders in childhood and adolescence may influence different aspects of socio-economic position, including educational attainment. We examined the association between early-onset disorders and subsequent educational achievement in a nationally representative sample of 4 351 South African adults. After adjusting for participant demographic characteristics and traumatic life events, post-traumatic stress disorder, major depressive disorder and substance-related disorders were each associated with increased odds of failing to complete secondary education (odds ratios and 95% confidence intervals 2.3, 1.0-5.1; 1.7, 1.2-2.5, and 1.7, 1.2-2.5, respectively). These data point to the role that early-life mental disorders may play in educational achievement and subsequent socio-economic position over the life course.