Mental illness is a leading cause of disability among youth. In Ontario, Canada, rates of mental health or addiction–related emergency department (ED) visits continue to rise in children and youth; however, it is unclear what is driving this change. We deconstructed this trend by sociodemographic and clinical characteristics, using linked health administrative data sets. Mental health or addiction–related ED visit rates increased by 89.1 percent between 2006 and 2017, with the greatest rise observed for those ages 14–21, high-acuity cases, and anxiety and mood disorders. We observed a significantly sharp increase after 2009, when several socioenvironmental changes occurred, including the emergence of social media and the Great Recession. Our findings of greater numbers of teenagers and young adults experiencing mental health problems and a shift in acuity and diagnoses have important implications for both ED staffing and outpatient mental illness prevention efforts. Further research is needed to examine whether better case management, care coordination, and after-hours services will help reverse these trends.