Background: Addiction medicine still largely neglects the topic of mild and borderline intellectual disabilities (MBID), although patients with MBID are considered a risk group for substance-related problems and offending behaviour. This study aimed to explore the cognitive and adaptive impairments of inpatients in forensic addiction mental health care. Method: Participants included a total of 50 consecutive referrals who had a substance use disorder. Results: 74% of the patients had a total IQ-score lower than 85, and 78% had an adaptive impairment in one or more domains of functioning. Only 10% had no cognitive or adaptive impairments. The total IQ-score significantly predicted the total number of impaired domains of adaptive functioning and the severity of substance use disorder did not predict adaptive functioning at all. Conclusions: It can be concluded that in forensic addiction care the prevalence of cognitive and adaptive impairments is high, with 62% having cognitive as well as adaptive impairments.