Background Little is known about the influence on child mental health symptoms of the timing of initial exposure to maternal major depression or whether the timing is associated with ‘pure’ or co-occurring internalising and externalising symptoms. Aims To address these issues, while also taking account of child gender and family socio-economic status. Method In a prospective community-based study, 421 kindergarten teachers rated children's symptoms. Previous assessments of maternal major depression indicated whether children were first exposed during infancy, in the toddler/pre-school period, or never. Results Exposure during infancy was associated with high internalising symptoms, especially when co-occurring with high externalising symptoms. Initial exposure in the toddler/pre-school years increased the risk of ‘pure’ externalising symptoms among girls. Conclusions The association of child mental health symptoms with the timing of initial exposure to maternal depression highlights the need for effective prevention and intervention strategies addressed to the developmental issues of each period.