Background Studies report that in the UK, among men with severe mental illness (SMI), those of black Caribbean ethnicity display increased risk of aggressive behaviour, criminal convictions, and schizophrenia. The study aimed to compare aggressive behaviour and criminal convictions among men with SMI of white British, black Caribbean and black African ethnicity, and to explore factors associated with differences across ethnicities. Method Sample 1 included 1,104 male inpatients with SMI. Sample 2 included a representative sub-sample of 165 who completed interviews, and authorized access to medical and criminal files. Ethnicity was self-ascribed. Results Staff-rated violence prior to admission, self-reported aggressive behaviour, and convictions for non-violent and violent crimes differed among men with SMI of different ethnicities. Relative to men with SMI of white British ethnicity, those of black African ethnicity showed decreased risk of aggressive behaviour, and those of black Caribbean ethnicity showed elevated risk of convictions for non-violent, and marginally, for violent crimes. Relative to men with SMI of black African ethnicity, those of black Caribbean ethnicity showed elevated risk of aggressive behaviour and criminal convictions. Proportionately more of the men of both black African and black Caribbean ethnicity, than those of white British ethnicity, presented schizophrenia spectrum disorders. Multivariate analyses failed to identify factors that would explain differences in aggressive behaviour, and criminal convictions across ethnic groups. Conclusions Differences in four different measures of aggressive and antisocial behaviour among men with SMI of different ethnicities were observed but factors associated with these differences were not found.