Background Recent research has suggested that too much self‐control (overcontrol) underpins a range of mental health disorders including certain Axis I mood and eating disorders, and Axis II presentations such as obsessive compulsive personality disorder and avoidant personality disorder. A novel group therapy intervention, Group Radical Openness (GRO), is proposed as a suitable treatment for overcontrol. Aims To explore whether appropriate participants for the intervention can be recruited; to investigate whether outcome measures are fit for purpose; to determine whether the intervention is suitable and acceptable to participants; and to determine whether GRO shows promise with the intended population. Method A mixed‐methods design was chosen, using statistical analysis on two primary measures and three secondary measures to assess pre–post outcomes and 6‐month follow‐up. This was combined with descriptive interpretive analysis of client interviews post‐intervention. Research participants were outpatients who attended GRO. The sample (N = 14; 6 females and 8 males; mean age 44.4 years; age range 18–58) attended 26, three‐hour group sessions. Results Quantitative data were analysed using repeated‐measures ANOVA. Participant scores on the two primary measures, The Five‐Factor Obsessive–Compulsive Inventory—Short Form (FFOCI‐SF) and the Brief Symptom Inventory (BSI), yielded significant results that were maintained at 6‐month follow‐up. Qualitative data found that participants reported changes across the three themes that are addressed in the GRO programme. Discussion This feasibility study found that GRO showed promise as a suitable intervention for individuals who are overcontrolled. Further intervention effectiveness research is warranted.