Background: People with intellectual disabilities have increasing difficulties managing their daily affairs. This study examined the effectiveness of a staff training, which teaches staff to promote self-management in people with intellectual disabilities. Method: Effectiveness was assessed with questionnaires addressing clients' (n = 26) independence and self-reliance, support needs and challenging behaviour, using a pre-posttest control group design. Additionally, focus groups were conducted with trained staff members 6 months after the training. Results: In the long term, the intervention group showed a significant increase in independence and self-reliance, in contrast to the comparison group. No effect was found on support needs and challenging behaviour. Trained staff members reported limited benefits of the training, but had noticed changes in their attitude and method of working afterwards. Conclusions: Further self-management research is required to investigate how independence and self-reliance can be promoted more effectively in this population. Future trainings should carefully consider their content, format, and implementation.