Mindfulness intervention aims to reduce stress and to improve physical and mental health. The present study investigated feasibility and effectiveness of mindfulness intervention in a prison context, in both a qualitative and quantitative fashion. Specifically, the mindfulness-based stress reduction (MBSR) intervention was investigated, in a retrospective pre–post design, in five Dutch prisons. Twenty-two inmates (out of 25 approached, mean age: 40.1 years ( SD = 11.1), convicted of murder, manslaughter, sexual offenses, drug offenses, robbery with violence, and/or illegal restraint/kidnap, and sentenced to incarceration between 15 and 209 months ( M = 5.5 years; SD = 3.8) took part in a semistructured interview after completion of the MBSR intervention. The interviews addressed level of satisfaction and challenges regarding the MBSR intervention as well as potential effects on stress responsivity, coping style, impulse control, aggression, and self-esteem. Ten staff members and four MBSR instructors were interviewed about their own practical issues experienced while providing or facilitating the MBSR intervention, and about the effects or changes they observed in the inmates who underwent the intervention. Both participants and instructors/prison staff reported improvements in all of the addressed domains and expressed satisfaction with the intervention. Challenges were mainly identified in practical issues regarding the organization of the intervention sessions. Future studies should investigate mindfulness in longitudinal randomly controlled designs, should strive for a multi-method approach, and distinguish inmates according to personality characteristics.