BACKGROUND: Sexual minorities suffer worse mental health than the sexual majority but little is known about differences in mental health within sexual minorities. We aimed to describe inequality in mental health indicators among gay and bisexual men. METHODS: Using multi-channel community-based opportunistic sampling we recruited 5799 eligible men aged 16 years and over, living in England, Scotland and Wales and who were sexually attracted to other men, to a self-completion Internet health survey. Mental health indicators (depression (PHQ-9), anxiety (GAD-7), suicide attempt and self-harm) were examined for independent associations across common axes of inequality (age, ethnicity, migrancy, education, income, cohabitation and living in London). RESULTS: Mental ill-health was common: 21.3% were depressed and 17.1% anxious, while 3.0% had experienced attempted suicide and 6.5% had self-harmed within the last 12 months. All four indicators were associated with younger age, lower education and lower income. Depression was also associated with being a member of visible ethnic minorities and sexual attraction to women as well as men. Cohabiting with a male partner and living in London were protective of mental health. CONCLUSION: Community interventions to increase mental health among gay and bisexual men should be designed to disproportionately benefit younger men and those living on lower incomes.