<p>Parental mental health has long-lasting impacts on children’s behavioural and emotional outcomes. Intergenerational transmission is particularly relevant to the child’s first 1,000 days of life (from conception to age 2 years). During this time, the foundations of biopsychosocial development are established, while the child is completely dependent on parental and caregiving figures. In this Review, we summarize systematic review and meta-analytic intergenerational mental health evidence focused on the first 1,000 days of life, considering vulnerability and resilience mechanisms that underpin intergenerational mental health and contextual factors (such as trauma). We shift the focus from a conceptualization of parent–child transmission centred around risk and mental health disorders to incorporate protective factors and positive outcomes towards a comprehensive understanding of the complex relationships between parental mental health and children’s behavioural, social and emotional development. We point towards opportunities to integrate findings about intergenerational mental health during the first 1,000 days of life into broader dimensions of research, practice and policy, such as preventative and early-intervention mental health approaches that support child and caregiver mental health across income settings.</p>