ObjectivesDuring COVID-19, sexual minority groups may have experienced heightened mental health challenges, amplified by unique stressors and the effects of pandemic-related restrictions. This study investigates the differential impact of the pandemic on mental health across sexual orientations, leveraging population-representative data to explore these disparities. Study designProspective cohort design. MethodsData from the United Kingdom Household Longitudinal Study (waves 8–12) was used. Monthly COVID-19 incidence rates at the regional level were used to indicate pandemic severity. Mental health outcomes were evaluated using the Mental Component Score of the Short Form-12 (MCS-12) survey. To examine whether COVID-19 led to differential impacts across sexual orientation, fixed-effect longitudinal models were employed, controlling for individual and time-variant covariates. ResultsLesbian women experienced a significant mental health decline during the COVID-19 pandemic, with a 3.10 MCS-12 score decrease (95 % confidence interval (CI): -5.77 to -0.43) and an 11.0 % higher likelihood of depression (95%CI: -0.3 % to 22.3 %, p = 0.057) compared to heterosexual women. Conversely, the impact on the mental health of heterosexual women was negative but not significant (-0.22; 95%CI: -0.47 to -0.04). Bisexual individuals and other women showed non-significant mental health declines. For men, COVID-19's effect on heterosexuals was similarly non-significant (-0.21; 95%CI: -0.48 to 0.1), with no significant differences observed in gay, bisexual, and other men. ConclusionsSexual minority individuals, especially lesbian women, faced heightened mental health challenges during COVID-19, emphasizing the urgency for targeted interventions.