BackgroundIndividuals with intellectual disabilities are at increased risk of mental health disorders, but may struggle to access appropriate services. While assessment/treatment may need to be adapted, knowledge is limited about what such adaptations may entail.MethodDuring a service development project, the participants (33 professionals, 13 family members) were asked to identify the characteristics/associated factors of individuals with intellectual disabilities and co‐occurring mental health disorders. Using thematic analysis, 1103 participant responses were analyzed to identify the needs for adaptation in assessment/treatment.ResultsThree core themes were identified: (1) A broader assessment: Gaining an accurate and comprehensive understanding of the individual's history, abilities and difficulties, (2) Adjusting/modifying existing treatment strategies, (3) Ensuring that the individual's day‐to‐day needs are met.ConclusionsAccording to the participants, assessment/treatment of mental health disorder in this population requires more time and broader assessments are necessary. Family/caregiver involvement and cross‐service organisation/collaboration represent other important adaptations.