Background Clinical trials and meta‐analyses provide some evidence for effectiveness of forgiveness therapy delivered individually or in groups. To date, however, forgiveness therapy has not been evaluated with mentally disordered offenders. Given the high prevalence of experienced and perpetrated trauma among such people, this population may particularly benefit from such an intervention. Aim The aim of this study is to test the feasibility and impact of a ‘learn to forgive’ group programme among mentally disordered offenders on a specialist secure hospital setting. Methods We conducted a non‐randomised trial with 36 offenders with mental disorders and 29 comparison patients. The intervention group engaged in a six‐week manual‐based ‘learn to forgive’ treatment programme, while the comparison group watched a 90‐minute video on forgiveness. Both groups completed measures of anger, depression, stress, forgiveness and satisfaction with life at baseline and then 6 and 18 weeks later. A repeated measures mixed‐effects model was used to investigate the association between affective outcomes and type of intervention received, after adjusting for baseline characteristics. Results The group completion rate was over 90%. The treatment and comparison groups were similar on baseline demographic and criminological measures, but the treatment group had higher baseline anger and depression scores. While both groups showed improved capacity to forgive and reduced negative affect over time, those in the ‘learn to forgive’ programme showed significantly more improvement in forgiveness and on anger measures. Conclusions and implications for practice Forgiveness training can be delivered effectively to offenders with mental disorders in clinical settings. Its range of benefits, including reduction I in anger as well as improved capacity to forgive, suggest that it may have longer term implications for personal safety and reintegration into mainstream societal settings. Copyright © 2016 John Wiley & Sons, Ltd.