Cognitive behaviour therapy (CBT) is an effective treatment for chronic fatigue syndrome (CFS; sometimes known as myalgic encephalomyelitis). However, only a minority of patients fully recover after CBT; thus, methods for improving treatment outcomes are required. This pilot study concerned a mindfulness-based cognitive therapy (MBCT) intervention adapted for people with CFS who were still experiencing excessive fatigue after CBT. The study aimed to investigate the acceptability of this new intervention and the feasibility of conducting a larger-scale randomized trial in the future. Preliminary efficacy analyses were also undertaken. Participants were randomly allocated to MBCT or waiting list. Sixteen MBCT participants and 19 waiting-list participants completed the study, with the intervention being delivered in two separate groups. Acceptability, engagement and participant-rated helpfulness of the intervention were high. Analysis of covariance controlling for pre-treatment scores indicated that, at post-treatment, MBCT participants reported lower levels of fatigue (the primary clinical outcome) than the waiting-list group. Similarly, there were significant group differences in fatigue at 2-month follow-up, and when the MBCT group was followed up to 6 months post-treatment, these improvements were maintained. The MBCT group also had superior outcomes on measures of impairment, depressed mood, catastrophic thinking about fatigue, all-or-nothing behavioural responses, unhelpful beliefs about emotions, mindfulness and self-compassion. In conclusion, MBCT is a promising and acceptable additional intervention for people still experiencing excessive fatigue after CBT for CFS, which should be investigated in a larger randomized controlled trial.