ObjectiveTo 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.MethodA 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.ResultsCompared 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.ConclusionThe 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.ConclusionThe 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.