Background: The reliability and validity of the Screener for Intelligence and Learning Disabilities (SCIL) are unknown in a population of outpatients with severe mental illness. The prevalence of mild or borderline intellectual disabilities (MBID); an umbrella term for people with borderline intellectual functioning (BIF) and mild intellectual disability (MID) in this population is also unknown. Methods: A total of 625 patients were screened with the SCIL, 201 of which also had IQ test results. Results: Cronbach's alpha of the SCIL was 0.73. The AUC value for detecting MBID was 0.81, and also 0.81 for detecting MID, with percentages of correctly classified subjects (when using the advised cut-off scores) being 73% and 79%, respectively. The SCIL results suggested that 40% of the patients were suspected of MBID and 20% of MID. Conclusion: The SCIL seems to be an appropriate screening tool for MBID. It is important to screen for MBID because a substantial proportion of outpatients with severe mental illness appear to be functioning at this level. It is necessary to adapt treatment for these patients.