Introduction: This article introduces the SAINT (Self-Assessment and INTervention), a guided self-help intervention for the treatment of mild depression in people with intellectual disabilities. Method: The study used a single-case experimental design and adopted quality frameworks specific to the approach to describe the participants and to standardize the study. The aim was to examine the acceptability and effectiveness of the SAINT on reducing symptoms of anxiety and depression. Semi-structured interviews were conducted to record user experiences and a framework-analysis approach was used. Attendance at sessions was also monitored. Results: Nine people receiving guided self-help using SAINT demonstrated a decrease in symptoms for both intervention phases for either depression or anxiety, with four showing a decrease in mean symptom scores in both intervention phases for both depression and anxiety. Those with a history of affective disorders and those with moderate intellectual disabilities showed improved symptom scores during both intervention phases. Summary and Conclusion: The idea that the SAINT can be feasibly implemented in routine clinical practice was broadly supported, with the positive outcomes relating to symptom reduction and acceptability. From the feedback received, the SAINT is tolerated well by participants and viewed positively by those using it and those who have supported people in its use.