PurposeYoung adults who self-identify as a sexual minority may have been particularly harmed by the consequences of lockdown, closure of educational institutions, and social distancing measures as they are likely to have been confined in households that may not be supportive of their sexual orientation. We examine inequalities in the mental health and self-rated health of sexual minority young adults, compared to their heterosexual peers, at the height of lockdown restrictions in the UK.MethodsWe analysed data from singletons who participated in waves 6, 7, and the wave 1 COVID-19 survey (n = 2211) of the Millennium Cohort Study, a nationally representative longitudinal study of infants born in the UK between September 2000 and January 2002. Regression models compared the mental health, self-rated health, and social support of sexual minority young adults to that of their heterosexual peers.ResultsOne in four young adults self-identified with a sexual orientation or attraction other than completely heterosexual. Sexual minority young adults had significantly lower levels of social support (β = − 0.38, SE 0.08), poorer self-rated health (OR 3.91, 95% CI 2.41–6.34), and higher levels of psychological distress (β = 2.26, SE 0.34), anxiety (β = 0.40, SE 0.15), and loneliness (β = 0.66, SE 0.18) when compared to heterosexual young adults.ConclusionsSexual minority young adults in the UK have been detrimentally impacted by the coronavirus pandemic, experiencing inequalities in mental health, self-rated health, and social support when compared to heterosexual young adults. Implications for policy and practice include a stronger provision of safe spaces in the community and in institutions, and policies that address marginalisation and harassment.