<p>Children may experience multiple adverse outcomes when a parent is incarcerated, and many incarcerated fathers experience parental stress. High parental stress negatively impacts wellbeing, parenting quality and prison adjustment. Despite evidence that maintaining parent-child relationships and fostering positive fathering identities can support desistance, understanding of these mechanisms remains limited, and more research is needed to evaluate the effectiveness of interventions which address these challenges in prison settings. This mixed-methods study aimed to evaluate <i>Fathers Inside</i>, a program aimed at developing positive fathering roles in prison. Twenty-seven fathers took part in the program, delivered in a UK prison. Their parental stress, wellbeing, judgmental attitudes, personal growth, locus of control, and self-esteem were measured before and after program participation. Pre- and post-program scores, and the Reliable Change Index were used to examine differences. While post-program scores showed no significant difference for any measures, on an individual level, several participants showed reliable change and clinical improvements using Reliable Change Index. Additionally, between 45 and 85% of participants’ scores were already comparable to the general population before commencement. Emerging themes in interviews included the reconfiguration of fatherhood roles from prison, allowing fathers to adapt to parenting at a distance, reframing fathering roles, accepting past mistakes, developing honesty as a value, and improving authentic communication with their children. The <i>Fathers Inside</i> program has the potential to help individuals reconstrue their fathering role in prison, reduce parental stress, thereby increasing adjustment to prison, and improve the parent-child relationship through authentic communication.</p>