Background: Individuals 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. Method: During 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. Results: Three 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. Conclusions: According 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.