The mental health of young people is declining globally (Benton et al., 2021). Australian national survey data show an approximate 50% increase in mental disorders among young people over the past two decades (Australian Bureau of Statistics [ABS], 2023), alongside an average annual rate of 3.6% increase in expenditure on child and adolescent mental health services (CAMHS) between 2015–2016 and 2019–2020 (Brazel et al., 2023). Adolescent mental illness and suicidality are strongly associated with child maltreatment (Angelakis et al., 2020; Baldwin et al., 2023), and self-harm and/or suicidal ideation are often recorded by child protection services prior to health services (O’Hare et al., 2023). Children exposed to severe maltreatment are more likely to be placed in out-of-home care (OOHC) and are at the highest risk of a range of poor outcomes in adulthood, including physical and mental health problems, homelessness and unemployment (Brännström et al., 2017). Here, we estimated the cumulative incidence of adolescent-onset mental disorders among youth aged 10–18 years who had been in contact with child protection services before age 10 years, and according to the highest level of child protection response which served as an indicator of the severity of maltreatment experienced (Mathews and Bross, 2015).