Physical assault during childhood is common and can lead to lasting mental health problems. Yet, there are few studies on the patterns of mental illness (ie, timing of onset, type, and acuity) in survivors of physical assault. To determine the risk of incident health record diagnoses of mental illness among children who experienced assault compared with children who did not. This population-based matched cohort study used linked health administrative data sets in Ontario, Canada. Children aged 0 to 13 years who experienced an incident physical assault between 2006 and 2014 were age-matched (1:4) to children who had not experienced assault and followed up for a minimum of 5 years. Data were analyzed from January 2020 to March 2022. Physical assault resulting in hospitalization or an emergency department (ED) visit between the ages of 0 and 13 years. The primary outcome was incident health record diagnosis of mental illness measured as any physician or hospital mental health care use or completed suicide. Secondary outcome measures included the acuity of incident mental illness and mental illness diagnostic category. Cox proportional hazards regression analysis generated hazard ratios (HR) for incident mental illness. A total of 21 948 children unexposed to assault and 5487 exposed to assault were included in the study with a mean (SD) age of 7.0 (4.6) years. There were more boys in the group that experienced assault (3006 individuals [54.8%]) compared with the group who did not (9909 individuals [45.1%]). Compared with children unexposed to assault, those exposed were more likely to be in the highest deprivation index quintile (standardized difference, 0.21) and live in rural areas (standardized difference, 0.48). Their mothers more often had active mental illness (standardized difference, 0.35). More than one-third of the exposed children had a health record diagnosis of mental illness (2219 children [38.6%]; incidence rate (IR), 53.3 per 1000 person-years) compared with 23.4% (5130 children; IR, 32.2 per 1000 person-years) of unexposed children, with an overall adjusted hazard ratio (aHR) of 1.96 (95% CI, 1.85-2.08). The greatest risk was observed in the first year following the assault (aHR, 3.08; 95% CI, 2.68-3.54). In both groups, nonpsychotic disorders were the most common type of mental illness. Initial mental illness diagnoses occurred in an acute care setting for 14.0% of exposed children (769 children) vs 2.8% of unexposed children (609 children). In this population-based matched cohort study, children who experienced assault had, on average, a 2 times higher risk of receiving a mental illness diagnosis and were more likely than children who had not experienced assault to present to acute care for mental illness. Early intervention to support mental health of assaulted children is warranted, particularly in the first year following assault.