While the prevalence of mental disorders in people with intellectual disabilities (ID) is well documented, there is less specific literature in the forensic domain. This study sought to clarify the psychiatric and criminological characteristics among Belgian French-speaker forensic inpatients with low IQ and mental health illnesses. To this end, we compared a low IQ group with mental health illnesses (n = 69), low IQ group (n = 56), and control group (n = 165). Compared with controls, proportionally more inpatients low IQ with Mental Health Illnesses presented a psychiatric illness, particularly a mood disorder, and proportionally fewer presented a cluster C personality disorder. The findings highlight the specificity and heterogeneity of the psychiatric profile of this subgroup of patients. He also demonstrated that forensic patients with ID are not a homogeneous group. This emphasizes the importance of considering in the management of forensic ID patients the specific needs with regard to their psychopathological profile.