To evaluate an emergency department (ED)-based mental health nurse practitioner (MHNP) outpatient service in Sydney, Australia. Data collection incorporated waiting times for follow-up outpatient appointments, two brief self-report measures (the K-10 measure of psychological distress and the General Self-Efficacy Scale), a satisfaction tool, and interviews conducted with a random selection of outpatients and a stratified, purposive sample of ED staff. Over 60% of outpatients were followed up within 5 days of their initial presentation. The mean K-10 score at baseline was 32 (very high psychological distress, n = 101) but this had decreased by two categories to 24 at follow-up (moderate psychological distress, n = 51). There was a modest association between decreased psychological distress and an increase in perceived self-efficacy. Participant satisfaction with aspects of the outpatient service was generally rated as high to very high. Interviewed outpatients (n = 23) were particularly positive about the accessibility, immediacy, and flexibility of the service and overall therapeutic benefits. Emergency staff (n = 20) considered the outpatient service enhanced service provision by facilitating access to a population of patients who were previously underserved. The ED-based MHNP role enhances access to specialized mental health care and also supports emergency staff.