BACKGROUND Begun in the late 1990s, mental health courts are specialty criminal courts developed to address the needs of persons with mental illness. METHODS As many persons with intellectual disabilities (IDs) may overlap in the mental health court system, we used mental health court records to examine the phenomenology and outcomes of 224 defendants with and without co-occurring IDs in the mental health court. This study had two goals: (1) to examine the prevalence of defendants with IDs in the court and (2) to compare defendants with dual diagnoses with defendants with lone mental health disorders. RESULTS Approximately 11% of defendants in the mental health court also had IDs. Compared with individuals with mental health disorders alone, individuals with dual diagnoses were more likely to be younger, male, African-American and less well-educated; these defendants were also more likely to show externalising, 'turning-against-others' symptoms, less likely to show internalising, 'turning-against-self' symptoms. Defendants with IDs (vs. those without) more often received behavioural, vocational rehabilitation and other services, although the two groups did not differ on most outcome variables. CONCLUSION Directions for future research are discussed.