Background: Sexual minority ([SM] or LGB+) adolescents experience mental health (MH) inequities and disproportionately high rates of bullying and victimisation. Less is known about the cumulative effects of bullying and victimisation across adolescence on MH in early adulthood, or the moderating role of loneliness and social support in this relationship. Objective: To investigate cumulative effects of bullying and victimisation in adolescence on MH in early adulthood, including the role of loneliness/social support. Methods: Drawing on a UK-wide nationally representative sample of individuals (N=12,872/LGB+ 22%) with self-reported bullying and victimisation assessed at ages 11/14/17/23, and MH (psychological distress/self-harm/attempted suicide) at age 23. Logistic models assessed separate associations between cumulative bullying or victimisation (0, 1, [≥]2times) and MH, and differences by sexual identity (using appropriate interactions), with adjustment for loneliness and social support. Findings: Across adolescence, SM individuals experienced higher prevalence of cumulative victimisation (3 times; gay/lesbian:22%, bisexual:22% vs heterosexual:9%) and bullying (2 times; gay/lesbian:35% and bisexual:27% vs heterosexual:18%). Models revealed substantial inequalities with higher proportions of SM individuals reporting self-harm compared to heterosexual peers with the same levels of experienced victimisation. Among those reporting victimisation 1 and [≥]2 times, 15% (95% CI 13-16%) and 23% (21-25%) of heterosexual individuals reported self-harm, increasing to 41% (33-50%) and 53% (47-59%) for bisexual, and 24% (10-39%) and 49% (40-59%) for gay/lesbian individuals, and 24% (11-38%) and 53% (39-67%) for other SM identities, respectively. Similarly, for heterosexual individuals experiencing victimisation 1 and [≥]2 times, 6% (5-8%) and 13% (11-14%) reported attempted suicide respectively. This increased to 21% (13-29%) and 38% (33-44%) for bisexual, and 17% (6-29%) and 34% (24-44%) for gay/lesbian individuals experiencing victimisation 1 and [≥]2 times, respectively. A key finding was that adjustment for loneliness and lack of peer/family support reduced the proportions reporting adverse mental health by 15-45% among SM but not heterosexual individuals. Similar patterns by sexual identity were found for bullying and all indicators of MH. Conclusions: SM individuals experience higher levels of cumulative bullying and victimisation across adolescence, which is associated with substantially higher rates of mental ill-health in early adulthood. Experiences of loneliness explain MH inequalities to greater extent in SM youth compared to heterosexual peers.