Background: Adolescents aged sixteen to eighteen years are mostly treated in adult emergency departments. In a study at our tertiary adult emergency department (ED) at the University Hospital in Bern (Inselspital), Switzerland, we found that adolescents significantly more often present with psychiatric problems than adults. The study at hand aimed to characterise those adolescents presenting to the ED triaged with a chief complaint regarding mental health. Furthermore, the goal was to assess sex differences in terms of diagnosis, suicidal ideation, substance use, as well as social impact. Methods: We conducted a single-centre, retrospective review of presentations to our adult ED related to the mental health of adolescents aged 16 to 18 years, covering the period from January 2013 to July 2017. Anonymised data were extracted from medical reports referring to the ED visits that were triaged as mental-health-related, and we assessed these for diagnosis, acute and previous suicidal ideation, history of self-harm, external aggression, substance use and social problems. We focused on patient characterisation and defining sex differences. Results: Data were analysed for a total of 612 consultations by adolescents who presented to our ED with problems related to mental health. Women accounted for 61.1% of cases. The most frequent diagnoses were reactions to severe stress and adjustment disorders (19.1%), followed by alcohol use disorders (17.6%), intentional self-harm (17.3%), and affective disorders (13.7%). Males had lower odds for intentional self-harm (OR 0.10, 95% 0.05–0.21, p < 0.001) as well as disorders of personality and behaviour (OR 0.09, 95% 0.21–0.37, p < 0.001), whereas they had higher odds of being admitted due to use of alcohol (OR 2.51, 95% 1.65–3.83, p < 0.001). Of all cases, 31.7% reported acute suicidal ideation, with a significantly lower odds ratio in males (OR 0.58, 95% 0.41–0.84, p = 0.004). The most common source for referral to the ED was family members (25.2%). Males had twice the odds of reporting alcohol consumption as well as use of cannabis (in both cases p < 0.001). In 27.9% of all cases, familial problems were reported, with males having significantly lower odds of being exposed to these (OR 0.64, 95% 0.44–0.94, p = 0.021), whereas they had higher odds of reporting educational problems (OR 1.68, 95% 1.04–2.72, p = 0.035). Conclusions: Adolescents aged sixteen to eighteen years presenting to the ED with problems related to mental health are an important subgroup of ED presentations and should be thoroughly assessed for suicidal ideation, substance use, as well as familial and educational problems. Assessment and establishment of post-ED care are of particular importance in this vulnerable patient group.