BackgroundTrauma and intellectual disability are highly prevalent in the serious mental ill (SMI). Little is known of their impact on general functioning and quality of life. AimThis study investigated the association of trauma and intellectual disability (ID) with general functioning and quality of life in SMI. MethodsPatient characteristics and diagnoses were extracted from electronic patient records. We used the Trauma Screening Questionnaire (TSQ), the Screener for Intelligence and Learning Disabilities (SCIL), the Health of the Nation Outcome Scale (HoNOS) and the Manchester Short Assessment of Quality of Life (MANSA) to asses trauma, intellectual impairment, general functioning and quality of life. Proportions on cut-off scores were analysed with cross-tabulations, questionnaire scores with t-tests. Multivariable associations were determined by logistic regression analysis. Results611 patients from an outpatient service were assessed. Trauma and ID were associated with each other (r = −0.207). Trauma was associated with worse general functioning and a lower quality of life. Mild intellectual disability (MID) or borderline intellectual functioning (BIF) were associated with worse general functioning. ConclusionsFor patients with SMI, trauma and ID should be identified early in care to treat the lower general functioning and quality of life it caused.