Background: Understanding the associations among mild intellectual disability (IQ 50-70), borderline intellectual functioning (IQ 70-85) and post-traumatic stress disorder (PTSD) with treatment outcomes in severely mentally ill patients could contribute to better tailoring of care. Methods: One or more assessments using the Health of the Nation Outcome Scale (HoNOS) were obtained from patients in long-term outpatient care. The Screener for Intelligence and Learning Disabilities (SCIL) was used to screen for mild intellectual disability and borderline intellectual functioning. The Trauma Screening Questionnaire was used to determine whether patients had PTSD symptoms. Results: Having more than six trauma-related symptoms and a SCIL score indicating an IQ between 70 and 85 were associated with less improvement on the HoNOS. No significant changes were found in patients with an estimated IQ below 70. Conclusions: This study confirms the value of screening for intellectual disability and trauma among severely mentally ill patients.