After completing treatment, childhood cancer survivors are at an increased risk for a lifetime of health problems, which can impair their quality of life (QOL). These issues emphasize the importance of identifying strategies that improve the QOL of childhood cancer survivors. There is consistent evidence that physical activity (PA) interventions can enhance QOL, however, few studies have focused on unstructured PA or investigated the underlying mechanisms. Understanding the mechanisms that explain how unstructured PA enhances childhood cancer survivors' QOL will further our knowledge and enable the refinement of sustainable PA interventions to promote optimal QOL. The objective of our longitudinal qualitative study was to explore how childhood cancer survivors' perceptions of QOL changed as a result of participating in unstructured recreational cycling for 3 months. We conducted semi-structured interviews with 4 childhood cancer survivors (Mage = 10.5 years; SD = 2.5) before they received a bicycle, and 4- and 8-weeks after. We analyzed the data using thematic analysis. Cycling enhanced participants' QOL over time by helping them: (a) Feel stronger and less tired, (b) Experience support from their social networks, and (c) Enhance feelings of self-efficacy and normalcy. Our study demonstrates that unstructured PA improves QOL in childhood cancer survivors. It also provides preliminary information about the mechanisms for how PA may promote these beneficial effects. If confirmed in larger studies, our findings suggest that PA interventions should explicitly aim to promote childhood cancer survivors' perceptions of their physical, psychological, and social functioning in order to optimize QOL.