The aim of this study is to examine age at menarche across sexual orientation groups. Data were obtained from 131,090 female participants, born 1947–2001, in 3 longitudinal studies—the Growing Up Today Study and Nurses’ Health Study 2 and 3. We estimated the association between sexual orientation and age at menarche using regression models adjusted for age, race/ethnicity, birthweight, height, and body mass index. Compared to heterosexual participants, sexual minorities were younger at menarche. Sexual minorities were more likely to have early menarche (≤11 years) and less likely to have late menarche (≥14 years) compared to heterosexual girls. As an example of this pattern, Nurses’ Health Study 3 bisexual participants were >30% more likely than heterosexuals to have early versus average menarche (odds ratio 1.37, 95% confidence interval 1.09–1.72). Sexual minority girls have a younger age at menarche than heterosexual girls and may benefit from screening for adverse outcomes associated with early menarche.