The aim of the current study was to identify the age at which sexual orientation disparity in mental health diagnoses can be first identified, as well as gender and sexual identity subgroup differences in such treatment, in a population-based sample. Young people aged 16 to 25 (N= 10,365) participating in the probability-based Swedish National Public Health Survey in 2018 were included. This sample was linked to physician-assessed mental health care treatment history data starting when all participants were 8 years old using national health care registries. Participants reporting a sexual minority identity in adolescence and young adulthood were more than 3 times as likely to have been treated for an internalizing disorder diagnosis (eg, depression, anxiety) and more than twice as likely to have been treated for a neurodevelopmental disorder diagnosis during childhood than participants reporting a heterosexual identity. Sexual minority participants overall and women in particular were more likely to have been treated for an internalizing disorder diagnosis at an early age compared with heterosexual participants, with this disparity starting at age 13. The sexual orientation disparity in likelihood of treatment for a neurodevelopmental disorder diagnosis was particularly elevated among bisexual/pansexual women with this disparity starting in early/middle adolescence. This population-based study linked to physician-assessed mental health diagnoses during childhood and adolescence identifies the age at which sexual orientation differences in treatment for common mental disorders emerge. The early emergence of this disparity suggests a potential benefit of interventions that facilitate social belonging for all youth.