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OBJECTIVES: Mental illness is known to adversely affect the physical health of patients in primary and acute care settings; however, its impact on critically ill patients is less well studied. This study aimed to determine the prevalence, characteristics, and outcomes of patients admitted to the ICU with a preexisting mental health disorder. DESIGN: A multicenter, retrospective cohort study using linked data from electronic ICU clinical progress notes and the Australia and New Zealand Intensive Care Society Adult Patient Database. SETTING/PATIENTS: All patients admitted to eight Australian adult ICUs in the calendar year 2019. Readmissions within the same hospitalization were excluded. INTERVENTIONS: None. MEASUREMENTS AND MAIN RESULTS: Natural language processing techniques were used to classify preexisting mental health disorders in participants based on clinician documentation in electronic ICU clinical progress notes. Sixteen thousand two hundred twenty-eight patients (58% male) were included in the study, of which 5,044 (31.1%) had a documented preexisting mental health disorder. Affective disorders were the most common subtype occurring in 2,633 patients (16.2%), followed by anxiety disorders, occurring in 1,611 patients (9.9%). Mixed-effects regression modeling found patients with a preexisting mental health disorder stayed in ICU 13% longer than other patients (β-coefficient, 0.12; 95% CI, 0.10–0.15) and were more likely to experience invasive ventilation (odds ratio, 1.42; 95% CI, 1.30–1.56). Severity of illness and ICU mortality rates were similar in both groups. CONCLUSIONS: Patients with preexisting mental health disorders form a significant subgroup within the ICU. The presence of a preexisting mental health disorder is associated with greater ICU length of stay and higher rates of invasive ventilation, suggesting these patients may have a different clinical trajectory to patients with no mental health history. Further research is needed to better understand the reasons for these adverse outcomes and to develop interventions to better support these patients during and after ICU admission. [ABSTRACT FROM AUTHOR]