Aim: Most evidence on sexual minority health focuses on quantifying mental health inequalities with limited evidence on differences in accessing and receiving mental healthcare. This study examined associations between sexual identity and self-reported seeking and receiving mental healthcare in young adults from a population-based cohort study. Methods: 12,782 individuals (22% LGBQ+) aged 23 from the UK wide Millennium Cohort Study. Associations between sexual identity (heterosexual [reference category], mainly heterosexual, gay/lesbian and other sexual identities[LGBQ+]) assessed at age 17 and seeking mental healthcare were examined using multivariable logistic regression with sequential adjustment for sociodemographic factors (sex/ethnicity/SES), past (ages 14-17) and current (age 23) mental health, victimisation, loneliness and social support. Associations between sexual identity and receiving mental healthcare at age 23 was examined using multivariable logistic regression among those who sought help. Predicted probabilities were estimated from logistic models for interpretation. Results: All LGBQ+ groups had higher odds of seeking mental healthcare than heterosexuals. In the minimally-adjusted model, predicted probability of seeking care was lowest for heterosexual (24%), highest for bisexual (45%) and gay/lesbian individuals (49%). These significant differences attenuated but persisted in fully adjusted models, narrowing the probability gap between heterosexual and LGBQ+ groups from 25% to 10%. Furthermore, mental health history independently predicted help-seeking behaviour at age 23 among heterosexuals; but not in LGBQ+ groups. Among individuals seeking support, LGBQ+ groups had higher odds of receiving care than heterosexual peers, with highest baseline probabilities observed in the other sexual identity group (35% vs 15%). These differences remained largely unchanged after adjustment. Conclusion: LGBQ+ groups face substantial disparities, demonstrating significantly higher odds of both seeking and receiving mental healthcare than heterosexual peers. Crucially, the elevated rates of receiving care persist after full adjustment, indicating that these inequalities remain largely independent of downstream mediators and mental health history.