OBJECTIVE: To explore mediated effects of Mindfulness-Based Cognitive Therapy-"Taking it Further" (MBCT-TiF) on mental well-being through changes in mindfulness, self-compassion, and decentering. METHOD: A secondary analysis of an RCT using simple mediation, with 164 graduates of MBCT and mindfulness-based stress reduction (MBSR), was implemented whereby MBCT-TiF (vs ongoing mindfulness practice; OMP) was the independent variable; changes in mindfulness, self-compassion, and decentering during the intervention were the mediators; and mental well-being at post-intervention, whilst controlling for baseline, was the dependent variable. Secondary outcomes included psychological quality of life, depression, and anxiety. RESULTS: Compared to OMP, MBCT-TiF experienced significant improvements in mental well-being through changes in all three mediators (mindfulness: ab = 0.11 [0.03, 0.25]; decentering: ab = 0.16 [0.05, 0.33]; self-compassion: ab = 0.07 [0.01, 0.18]). A similar pattern was demonstrated for depression, but only mindfulness and decentering mediated effects on psychological quality of life and anxiety. CONCLUSION: The findings provide preliminary support for all three mediators in driving change in mental well-being in a sample of MBCT/MBSR graduates. Future work must be theory-driven and powered to test all mediators in parallel and alongside other potential mediators (e.g., equanimity) to further understand independent contributions and interacting effects.Trial registration: ClinicalTrials.gov identifier: NCT05154266.