Introduction: About one‐third of cancer survivors suffer from severe chronic fatigue. Aim of this study was to evaluate the efficacy of mindfulness‐based cognitive group therapy in reducing severe chronic fatigue in cancer survivors with mixed diagnoses.Patients and Methods: Participants (n = 100) were randomly selected from a cohort and allocated to an intervention and a waiting list condition. Analyses were based on 59 participants in the intervention condition and 24 in the waiting‐list condition. Fatigue severity (Checklist Individual Strength), functional impairment (Sickness Impact Profile) and well being (Health and Disease‐Inventory) were assessed before and after the 9‐week intervention. The intervention group had a follow‐up 6 months following the intervention. Results: At post‐treatment measurement the proportion of clinically improved participants was 30%, versus 4% in the waiting list condition (χ2 (1) = 6.71; p = 0.007). The mean fatigue score at post‐measurement was significantly lower in the intervention group than in the waiting list group corrected for pre‐treatment level of fatigue. The mean well‐being score at post‐measurement was significantly higher in the intervention group than in the waiting list group corrected for pre‐treatment level of well‐being. The treatment effect was maintained at 6‐month follow‐up. No difference between the two conditions was found in functional impairment.