Introduction: People arriving to the emergency department with mental health problems experience varying and sometimes inferior outcomes compared with people without mental health problems, yet little is known about whether or how their arrival mode is associated with these outcomes. This study describes and compares demographics, clinical characteristics, and patient and health service outcomes of adult mental health emergency department patient presentations, based on arrival mode: brought in by ambulance, privately arranged transport, and brought in by police. Methods: Using a retrospective observational study design with state-wide administrative data from Queensland, Australia, mental health presentations from January 1, 2012, to December 31, 2017, were analyzed using descriptive and inferential analyses. Results: Of the 446,815 presentations, 51.8% were brought in by ambulance, 37.2% arrived via privately arranged transport, and 11.0% were brought in by police. Compared with other arrival modes, presentations brought in by ambulance were more likely to be older and female and have more urgent triage categories and a longer length of stay. Presentations arriving by privately arranged transport were more likely than other arrival modes to present during the day, be assigned a less urgent triage category, be seen within their recommended triage time, have a shorter length of stay in the emergency department, have higher rates of discharge, and have waited longer to be seen by a clinician. Presentations brought in by police were more likely than other arrival modes to be younger and male and experience a shorter time to be seen by a clinician. Discussion: Discrepancies between arrival modes indicates a need for further investigation to support inter- and intra-agency mental health care interventions.