Abstract Purpose The Myeloma: Advancing Survival Cancer Outcomes Trial (MASCOT) tested the impact of a supervised exercise programme on fatigue, clinical, and patient-reported outcomes in multiple myeloma [MM] patients. The current study explored MM patients’ experiences of the programme to guide future interventions. Methods Purposive sampling was used to recruit stable MM patients participating in MASCOT. Semi-structured, face-to-face interviews were conducted, transcribed verbatim, and analysed using thematic analysis. Results Six themes were identified. Key drivers for participation in MASCOT were “Altruism and extended cancer care”; participants wanted to give something back and assist in improving post-treatment care for MM patients, especially as after treatment “Barriers to being physically active” were a fear of damage and lack of health professional guidance. “Influences fostering change within the intervention” included physiotherapy supervision and tailored exercises, which gave participants confidence to push themselves in a safe environment and broke down misconceptions about their body. “Social support”, from both family and peers in the programme, promoted motivation and adherence. Participants expressed concerns about “Maintaining things going forward” but had identified mechanisms to aid continuation. “Physical and mental benefits” of the programme were highlighted; participants were able to do things they couldn’t before and described feeling free from the constraints of MM. Conclusions A post-treatment exercise intervention for MM patients was a positive experience, which enhanced participants’ physical and psychological wellbeing. Tailored gym and home-based exercises, a specialist cancer physiotherapist, and sustained support were perceived to be important for success. Implications for cancer survivors Exercise support for MM patients, ideally with physiotherapist supervision, should be incorporated into survivorship care to qualitatively improve patients’ quality of life, self-efficacy, and mental wellbeing.