OBJECTIVE: To evaluate the effectiveness and acceptability of Mindfulness-Based Cognitive Therapy-Taking it Further (MBCT-TiF), as an adapted programme for graduates of MBCT and Mindfulness-Based Stress Reduction (MBSR). MBCT-TiF sits within a global mental health approach, which aims to help shift a wider distribution of the population towards mental well-being and away from mental ill health using a family of MBCT curricula. The primary hypothesis was that MBCT-TiF, compared to Ongoing Mindfulness Practice (OMP), would help MBCT/MBSR graduates improve their mental well-being. METHOD: A parallel RCT with repeated measures was conducted. 164 graduates of MBCT/MBSR were randomly assigned (1:1) to either MBCT-TiF or OMP. REGISTRATION: ClinicalTrials.gov (NCT05154266). RESULTS: Of the 164 graduates recruited, 83 were randomly assigned to MBCT-TiF and 81 to OMP. MBCT-TiF was significantly more effective than OMP at improving mental well-being, with large effects post-intervention (B = 6.25; 95% CI = [4.20, 8.29]; Cohen's d = 0.78). No serious adverse effects were reported. CONCLUSIONS: The findings support MBCT-TiF, in the context of the proposed global mental health approach, to help MBCT/MBSR graduates sustain mental health benefits and experience further gains in mental well-being after completing an introductory MBCT/MBSR programme. Future work should consider mechanisms and longer follow-up measurements.