Background: A growing literature suggests that people with mild intellectual disability (ID) who have depressed mood may benefit from cognitive--behavioural interventions. There has been some speculation regarding the relative merit of the components of this approach. The aim of this study was to compare (i) cognitive strategies; (ii) behavioural strategies; and (iii) combined cognitive-behavioural (CB) strategies on depressed mood among a sample of 70 individuals with mild ID. Methods: Staff from three participating agencies received training in how to screen individuals with mild ID for depressive symptoms and risk factors for depression. Depressive symptoms and negative automatic thoughts were assessed prior to and at the conclusion of the intervention, and at 6-month follow-up. The interventions were run in groups by the same therapist. Results: A post-intervention reduction in depression scores was evident in participants of all three interventions, with no significant difference between groups. A significant reduction in negative automatic thoughts post-intervention was evident in the CB combination group and was maintained at follow-up. Examination of clinical effectiveness suggests some advantage of the CB combination in terms of improvement and highlights the possible short term impact of behavioural strategies in comparison with the longer-term potential of cognitive strategies. Conclusions: The findings support the use of group cognitive-behavioural interventions for addressing symptoms of depression among people with ID. Further research is necessary to determine the effectiveness of components.