Abstract Background Previous studies report vast mental health problems in sexual minority people. Representative national proportion estimates on self-identifying LGB+ persons are missing in Belgium. Lacking data collection regarding sexual orientation in either census or governmental survey data limits our understanding of the true population sizes of different sexual orientation groups and their respective health outcomes. This study assessed the proportion of LGB+ and heterosexual persons in Belgium, LGB+ persons’ self-identification as sexual minority, mental health, and experienced minority stress. Method A representative sample of 4632 individuals drawn from the Belgian National Register completed measures of sexual orientation, subjective minority status, and its importance for their identity as well as a range of mental-health measures. Results LGB+ participants made up 10.02% of the total sample and 52.59% of LGB+ participants self-identified as sexual minority. Most sexual minority participants considered sexual minority characteristics important for their identity. LGB+ persons reported significantly worse mental health than heterosexual persons. Sexual minority participants did not report high levels of minority stress, but those who considered minority characteristics key for their identity reported higher levels of minority stress. LGB+ participants who did not identify as minority reported fewer persons they trust. Conclusions The proportion of persons who identified as LGB+ was twice as large as the proportion of persons who identified as a minority based on their sexual orientation. LGB+ persons show poorer mental health compared to heterosexual persons. This difference was unrelated to minority stress, sociodemographic differences, minority identification, or the importance attached to minority characteristics.