Background <p>People attending the Emergency Department (ED) with mental health conditions are a vulnerable group who may struggle to engage with scheduled care. Existing research suggests this cohort are more likely to leave before treatment is complete. This study aimed to examine the utilisation rates of pre-hospital emergency services in transferring people with mental health triage codes to the ED, establish rates on incomplete care, and examine any associations between mode of arrival and self-discharge prior to completion of care.</p> Methods <p>This study utilised retrospective analysis of attendances at an inner-city ED on individuals presenting with a mental health triage code: mental illness; behaving strangely; overdose &amp; poisoning; and self-harm. SPSS was used to perform descriptive, bivariate, and multivariate analyses of patient characteristics and the associations between mode of arrival and self-discharge from 2008 to 2022.</p> Results <p>During the study period 30,941 patients presented to the ED with mental health triage codes. Of these, 53% (<i>n</i> = 16,390) were brought by ambulance and 3.3% (<i>n</i> = 1,016) by the police service. In total, 28.4% (<i>n</i> = 8,732) left before completion of care. Self-discharge was significantly associated with arrival by ambulance (<i>p</i> &lt; 0.001), less urgent triage categories (<i>p</i> &lt; 0.001), and male gender (<i>p</i> &lt; 0.001). The association between self-discharge and mode of arrival remained significant after controlling for age and gender, triage code and time of arrival (OR = 1.42; <i>p</i> &lt; 0.001).</p> Conclusion <p>In people attending the ED with mental health problems, self-discharge is more frequent in those brought by ambulance. There is a need for qualitative data to understand the needs of this population and target interventions to avoid premature cessation of treatment.</p> Clinical trial number <p>Not applicable.</p>