Purpose: This study examines the relationship between mental illness diagnoses and four intergenerational patterns of child protection services involvement: cycle breakers, cycle maintainers, cycle initiators, and a comparison group (no maltreatment). Existing research is limited and inconsistent, and rarely incorporates multiple categories of mental illness or considers variation between mental illnesses. Methods: Data were drawn from an administrative population-based data repository in Queensland, Australia and includes 32,494 individuals identified as biological parents. Child protection data were obtained from the Department of Children, Youth Justice and Multicultural Affairs and mental illness diagnoses were obtained from Queensland Health hospital admissions. Any mental illness diagnosis, age at onset (adolescence or adulthood), and diagnosis types (common, severe, personality disorders, childhood-onset, adolescent- and adult-onset, and substance use) were examined. Multinomial and logistic regressions were conducted to investigate whether the mental illness diagnosis variables distinguished the four intergenerational patterns of child protection service involvement. Results: Overall, 10.4% of individuals had at least one hospital admission involving a mental illness diagnosis. The prevalence of mental illness diagnoses significantly differed across the intergenerational patterns. Cycle maintainers and cycle initiators received the highest rates of diagnoses (50% and 38.8%, respectively), compared to cycle breakers (21.1%) and the comparison group (7.7%). Conclusions: Our findings underline the need for early access to mental health supports for families involved with the child protection system, which could help prevent the cycle of maltreatment.