PURPOSE: To investigate the extent to which different trajectories of adverse childhood experiences (ACEs) between the prenatal period and middle childhood are associated with suicidal ideation and self-harm during adolescence. METHODS: Participants were 73,833 children followed from birth to ∼18 years within an Australian population cohort study (the NSW Child Development Study) conducted via multiagency record-linkage. Service contacts for self-harm/suicidal ideation (≥12 years) were identified in health (inpatient and outpatient) and nonhealth (child protection and police) administrative records. Indices representing 17 ACEs were derived from parent and child records for the prenatal, early childhood, and middle childhood developmental periods (<12 years). Trajectories of cumulative ACE scores were identified using k-means clustering for longitudinal data. RESULTS: Six trajectories of ACE exposure were identified, representing differences in the severity and persistence of adversity experienced across development. Approximately 73% of children with a record of self-harm/suicidal ideation had elevated ACEs in at least one developmental stage, and around 13% of the sample (representing two trajectories) had high ACE exposure in early-middle childhood. All five trajectories representing elevated ACE exposure were associated with self-harm/suicidal ideation, with particularly strong associations when ACE scores were persistently elevated across early-middle childhood. DISCUSSION: Persistent exposure to adversity across early and middle childhood is strongly associated with self-harm/suicidal ideation in adolescence. Engagement with multiple government agencies provides an opportunity for the early detection of youth at risk of self-harm and suicide, potentially via increased mental health monitoring by (nonhealth) service agencies (e.g., police, child protection, and welfare).