Although there has been a lot of transformation in prisons over time in pursuit of improved outcomes, many prisoners continue to be released with psychological problems they developed in prison that were never properly addressed. This increases their risk of re-offending and makes it harder to reintegrate in the society as a productive, law abiding citizen. More than half of people who have been incarcerated are arrested again within three years; and the state of mental wellbeing of inmates within prison contributes immensely to this high rate of re-offending. Despite various intervention programs being initiated by prisons, there are still many cases of mental disorders and recidivism in prisons. In view of this, this study sought to determine the influence of rehabilitation programs on the mental wellbeing of prisoners in Kenyan prisons. It specifically aimed at determining the level of mental wellbeing of prisoners, the level of influence of rehabilitation programs on the mental wellbeing of prisoners and to identify the determinants of the influence of rehabilitation programs on the mental wellbeing of prisoners. The research employed concurrent triangulation research design. The study population was 364 respondents from two selected prisons. A structured questionnaire, MINI International Neuropsychiatric Interview and Warwick-Edinburg Mental Wellbeing Scales were used to screen prisoners, detect certain mental disorders and measure the wellbeing respectively. All the tools had high validity and reliability scores. The collected data was processed and analysed using SPSS version 22. The analysis sought to determine differences between groups using chi-square test. Comparison of groups was done using independent sample t-test and ANOVA. A p-value <0.05 with 95% confidence intervals was considered a statistically significant level of precision. The data is presented using frequencies, means, percentages and standard deviation. The level of mental wellbeing of prisoners is high with 86.3% having positive mental wellbeing scores. There is a high prevalence of mental disorders among the prisoners at 63.2% with almost equal distribution between male and female prisoners’. Mental disorders influence the mental wellbeing of prisoners with suicidality, panic disorder, PTSD and mood disorder with psychotic features significantly reducing mental wellbeing at p-values = 0.001, 0.015, 0.044 and 0.019 respectively. Rehabilitation programs have a significant positive influence on the level of mental wellbeing. Rehabilitation programs do not reduce the risk of mental disorder. Uptake of rehabilitation programs is high at 89.8%, with more female (96.7%) than male (88%) prisoners participating. Reasons for non-uptake of programs were prison policy, despair due to long or life prison sentences, feeling of certificates being useless due to prison rubber stamp, indecision on choice of program, inadequate resources and wrong attitude. Majority of the prisoners have a positive attitude towards the rehabilitation programs with 92.3% willing to participate. Age, level of education of prisoner, professional training of prison staff and degree to which the program met offender needs were determinants of the influence of a rehabilitation program on the mental wellbeing of a prisoner. The study concluded that level of positive mental wellbeing among prisoners was high. Rehabilitation programs had a positive influence on the mental wellbeing of prisoners, though the effect was limited. Uptake of rehabilitation programs was high. This study recommends that the Kenya Prison Service management should develop an assessment tool to determine the prisoner needs in order to guide them appropriately; and also partner with industries to get a labour market. The respective prisoner managers should promote a change in attitude of prison staff towards the prisoners.