It has been suggested that patients with mental illness wait longer for care than other patients in the emergency department. We determined wait times for patients with and without mental health diagnoses during crowded and noncrowded periods in the emergency department.We conducted a population-based retrospective cohort analysis of adults seen in 155 emergency departments in Ontario between April 2007 and March 2009. We compared wait times and triage scores for patients with mental illness to those for all other patients who presented to the emergency department during the study period.The patients with mental illness (n = 51 381) received higher priority triage scores than other patients, regardless of crowding. The time to assessment by a physician was longer overall for patients with mental illness than for other patients (median 82, interquartile range [IQR] 41-147 min v. median 75 [IQR 36-140] min; p < 0.001). The median time from the decision to admit the patient to hospital to ward transfer was markedly shorter for patients with mental illness than for other patients (median 74 [IQR 15-215] min v. median 152 [IQR 45-605] min; p < 0.001). After adjustment for other variables, patients with mental illness waited 10 minutes longer to see a physician compared with other patients during noncrowded periods (95% confidence interval [CI] 8 to 11), but they waited significantly less time than other patients as crowding increased (mild crowding: -14 [95% CI -12 to -15] min; moderate crowding: -38 [95% CI -35 to -42] min; severe crowding: -48 [95% CI -39 to -56] min; p < 0.001).Patients with mental illness were triaged appropriately in Ontario's emergency departments. These patients waited less time than other patients to see a physician under crowded conditions and only slightly longer under noncrowded conditions.