These case studies examined the effects of mindful sport performance enhancement (MSPE) programs on competitive swimming performance, flow experience, and emotional regulation. Sixteen national competitive adolescent swimmers were randomly allocated into MSPE (n = 9) and relaxation training (n = 7) groups for 8 weeks. In the second phase of the intervention, the 16 participants were evenly split into an MSPE group for swimming and a control group for 5 weeks. The participants completed measures of trait and state flow, and mindfulness pre- and postintervention. The data analysis revealed that mindfulness had significant effects on both action–awareness merging and clear goals subscales. Further analysis revealed that the MSPE for swimming group significantly improved in terms of presence relative to the control group. In conclusion, general and sport-specific mindfulness interventions can psychologically benefit adolescent competitive swimmers.