Introduction: Intellectual disability is a key subject in all mental healthcare institutions, including the forensic mental health services. The Screener for Intelligence and Learning Disability (SCIL) is designed to screen for intellectual disability in forensic populations. So far, this assessment method is only validated in “detention fit prisoners” with low need of care. The aim of the present study was to investigate the psychometric properties of the SCIL in a population of mentally ill detainees with high need of care. Materials and methods: Screener for Intelligence and Learning Disability scores, mental health reports including intelligence assessment, and criminal records of detainees were obtained. Reliability and validity of the SCIL were calculated, using the Wechsler Adult Intelligence Scale-Fourth Edition (WAIS-IV) and prior studies of the SCIL as a reference. Results: Cronbach's alpha coefficient for the total SCIL was 0.72. The area under the receiver operating characteristics (ROC) curve was 0.84. Different cut-off values than the original were determined when achieving the optimum in true positives and negatives. The mean intelligence quotient (IQ) score of the study population was 82.6, and 60.3% could be classified with an IQ < 85. Discussion: The SCIL gives a quick and accurate indication of whether a person is at risk for intellectual disabilities. Although both the reliability and validity of the SCIL are lower in the study population than in regular prison populations, for application of the SCIL in mentally ill detainees all psychometric properties could be classified as acceptable. When assessing the latter populations, it is recommended to use a cut-off value of 20.5 instead of the original 19.5. Additionally, it is advised to revise item 4. Finally, it is important to mention that the estimated prevalence of intellectual disability is the forensic population seems to be larger than expected.