Background Adults with mild intellectual disability (MID) experience more mental health disorders than the general population. However, mental healthcare may be insufficiently tailored to match their needs. Detailed information is lacking regarding care provided to people with MID in mental health services. Aims To compare mental health disorders and care provided to patients with and without MID in Dutch mental health services, including patients with missing MID status in the service files. Method In this population-based database study, we used a Statistics Netherlands mental health service database, containing health insurance claims of patients who utilised advanced mental health services in 2015–2017. Patients with MID were identified by linking this database with Statistic Netherlands’ social services and long-term care databases. Results We identified 7596 patients with MID, of whom 60.6% had no intellectual disability registration in the service files. Compared with patients without intellectual disability (n = 329 864), they had different profiles of mental health disorders. They received fewer diagnostic (odds ratio 0.71, 95% CI 0.67–0.75) and treatment activities (odds ratio 0.56, 95% CI 0.53–0.59), and required more interprofessional consultations outside of the service (odds ratio 2.06, 95% CI 1.97–2.16), crisis interventions (odds ratio 2.00, 95% CI 1.90–2.10) and mental health-related hospital admissions (odds ratio 1.72, 95% CI 1.63–1.82). Conclusions Patients with MID in mental health services have different profiles of mental health disorders and care than patients without intellectual disability. In particular, fewer diagnostics and treatments are provided, especially in those with MID with no intellectual disability registration, putting patients with MID at risk of undertreatment and poorer mental health outcomes.