Introduction: Autistic people with co-occurring intellectual disability have a high risk of mental disorders. This study aimed to investigate specialized mental health services for this group, emphasizing inpatient/outpatient treatment, patient characteristics, interventions, and outcomes. Methods: The current study used data from a Norwegian multicenter study (the AUP study), 151 participants were included. Statistical analyses included comparisons of the two treatment groups and multiple analyses of variance on outcome measures of behavioral and mental health symptoms for all patients. Results: Seventy-four patients received outpatient treatment and 77 received inpatient treatment. The inpatients had significantly more "severe" mental disorders and received a higher number of interventions. Both groups improved significantly during treatment, inpatients slightly more. Psychotherapy was associated with positive outcomes. Psychopharmacological treatment or duration were not associated with outcomes. Conclusion: Treatment of mental disorders is effective in autistic people with co-occurring intellectual disability. Psychotherapy may be beneficial for these patients.