Sexual minorities, including children, are at increased risk for adverse mental health outcomes compared to their heterosexual peers, but longitudinal studies are needed to determine the factors that explain the associations between sexual minority identification and adverse mental health outcomes during this developmental period. We examined longitudinal associations between sexual orientation and mental health over 2 years in a US cohort of children (aged 9-10 at baseline) and two explanatory factors (increased social problems such as getting teased and decreased perceived school safety). We hypothesized that beginning to identify as gay/bisexual and consistently identifying as gay/bisexual would be associated with increases in internalizing (e.g. depression, anxiety) and externalizing (e.g. aggression) problems compared to consistently identifying as heterosexual, and these associations would be partially explained by increased social problems and decreased perceived school safety. We used data from Waves 1-3 of the Adolescent Brain Cognitive Development study. The analytic sample included 5,574 children (46.0% female; 55.1% non-Hispanic White). Beginning to identify as gay/bisexual was associated with increased internalizing/externalizing problems, and consistently identifying as gay/bisexual was associated with increased internalizing problems, compared to consistently identifying as heterosexual. For those who consistently identified as gay/bisexual, increased disparities in internalizing problems were partially explained by increased social problems and decreased perceived school safety, and increased disparities in externalizing problems were partially explained by increased social problems. These findings suggest the health disparities affecting sexual minority children include both internalizing and externalizing problems, and social problems and feeling unsafe at school may be contributing factors.