In a large (n = 10 103), nationally representative sample of 17-year-olds 16.1% had experienced high psychological distress in the past 30 days, 24.1% had self-harmed in the previous 12 months and 7.4% had ever attempted suicide. Females, White adolescents, sexual minorities and those from more socioeconomically disadvantaged families had worse mental health outcomes; with the exceptions of no detected differences in attempted suicide by ethnicity and in self-harm by socioeconomic position. Findings include a narrower gender gap in self-harm at age 17 (males 20.1%, females 28.2%) compared with at age 14 (males 8.5%; females 22.8%) and 2-4 times higher prevalence in sexual minority adolescents (39.3% high distress, 55.8% self-harmed, 21.7% attempted suicide compared with 13.4%, 20.5% and 5.8%, respectively, in heterosexual adolescents).