People with disabling long-term conditions often require rehabilitation to help them function optimally in society. Rehabilitation is by definition a complex intervention, meaning multiple factors can influence the outcomes we see following treatment. With multiple factors at play, trying to understand the effects rehabilitation treatments have on people is challenging. This matters clinically, when considering the effectiveness of existing treatments, but also academically, when designing new interventions. The UK Medical Research Council has a framework to help guide researchers through the process of designing and evaluating complex interventions. We used this to evaluate the potential use of a mindfulness-based intervention for people with multiple sclerosis. This case describes that process, our experiences, and findings. We used a mixed-methods approach, including quantitative epidemiological work, a systematic review and meta-analysis, and a feasibility randomized controlled trial, complemented by qualitative interview data from study participants to help us better understand why we were seeing what we were, and help us improve the mindfulness intervention, making it more relevant and accessible to those taking part. We found a clear need for novel and effective psychosocial interventions to help people with multiple sclerosis deal with stress and disability. We also identified that no clear optimal mindfulness-based intervention approach exists in this context. We thus tested a standard mindfulness-based intervention in a group of people with multiple sclerosis and used their feedback to improve the course. The next stage for this work would be to test the optimized intervention in a full-scale randomized controlled trial.