OBJECTIVE: To examine factors associated with high intensity physician-based mental health care services in a population-based sample of children and youth in Ontario, Canada. METHODS: Data from the 2014 Ontario Child Health Study (OCHS) were linked at the person-level to longitudinal health administrative databases containing physician contacts in outpatient settings, emergency departments and hospitals. Our analytical sample (15.8% of 9,301, RESULTS: Among those with at least one contact, being older (PR: 1.15, 95% CI: 1.04, 1.25), having more severe symptoms of mental ill-health (PR: 1.04, 95% CI: 1.01, 1.06) and having a history of mental health service use (PR: 3.99, 95% CI: 1.37, 11.61), were positively associated with high-intensity service use, while living in a rural setting (PR: 0.35, 95% CI: 0.15, 0.30) was negatively associated. Findings were largely consistent between the top 10th and fifth percentiles. Notably, among youth ages 14-17 years, self-reported prior suicide attempt was positively associated with high-intensity (top fifth percentile) service use (PR: 6.09, 95% CI: 1.41, 26.26). CONCLUSIONS: Our findings suggest older age, non-rural residency, mental health symptom severity and suicidal behaviour are important factors associated with high-intensity physician-based mental health service use. Our findings will inform efforts to better identify children and youth who may benefit from early and personalized interventions. PLAIN LANGUAGE SUMMARY TITLE: Understanding Children and Youth with the Greatest Mental Health Related Service Needs.