During the potentially tumultuous adolescent period, non-suicidal self-injury (NSSI) and suicide attempts are relatively common, particularly amongst youth who present to mental health services. These phenomena frequently co-occur but their relationship is unclear. This study evaluated clinical data from 468 youth between the ages of 12 and 17 years (63.5% female) to determine the incidence of NSSI 24 h prior to presentation at emergency crisis services, evaluated the overlap between NSSI and suicide attempt, and examined the characteristics of different types of self-harm. Half of the adolescents presenting to emergency crisis services had self-harmed within the previous 24 h, with most of these (91%) classified as NSSI only. The percentage of youth with a suicide attempt was 5% and the co-occurrence of these two behaviours was 4%. Group differences in depressive symptoms, suicidal ideation and impulsivity were identified, with the co-occurring NSSI and suicide attempt group presenting with the highest level of psychopathology. This study underscores the necessity of assessing suicidal ideation and NSSI in all youth presenting to mental health services.