BACKGROUND: Adolescent suicidality, suicidal ideation (SUI) and self-harming behaviour (SI) are major public health issues. One group of adolescents known to be particularly prone to suicidality and mental health problems is lesbian, gay and bisexual (LGB) youth. Although the social acceptance of the LGB community has increased in recent years, LGB individuals are still at risk of mental health issues and suicidal behaviour. More longitudinal research looking into the associations between sexual orientation (SO) and facets of mental health across adolescence is warranted. METHODS: This research examined associations between sexual orientation, suicidal ideation and self-injury at 15, 17 and 20 years of age in a community-based sample of 1108 Swiss adolescents (51.1% females/48.9% males). At the age of 15 years, participants provided information regarding their SUI and SI. At 17 and 20 years of age, participants also reported their SO. RESULTS: Twelve percent of the female participants and 4.4% of the male participants reported identifying as LGB at 17 and 20 years of age. Self-reports of bi- or same-sex attraction increased over time in both genders, with the increase being more pronounced in females. LGB adolescents of both genders showed significantly higher percentages of SUI and SI at the ages of 17 and 20 years than their heterosexual peers. CONCLUSIONS: The findings confirm a higher risk of SUI and SI in adolescents who identify as LGB. Future studies should develop interventions targeting mental health from early adolescence with the aim of reducing disparities related to SO.