OBJECTIVE: This study is a secondary analysis evaluating changes in cognitive fusion and pain catastrophizing over 8 weeks of mindfulness-based cognitive therapy for migraine (MBCT-M) intervention versus waitlist/treatment as usual. BACKGROUND: Migraine is a common disabling neurological condition. MBCT-M combines elements of cognitive behavioral therapy with mindfulness-based approaches and has demonstrated efficacy in reducing migraine-related disability. METHODS: A total of 60 adults with migraine completed a 30-day run-in before randomization into a parallel design of either eight weekly individual MBCT-M sessions (n = 31) or waitlist/treatment as usual (n = 29): participants were followed for 1 month after. Participants completed the Pain Catastrophizing Scale (PCS) and the Cognitive Fusion Questionnaire (CFQ) at Months 0, 1, 2, and 4. RESULTS: The PCS scores decreased more in the MBCT-M group (mean [SD] at baseline = 22.5 [9.6]; at Month 4 = 15.1 [8.8]) than in the waitlist/treatment as usual group (mean [SD] at baseline = 24.9 [9.0]; at Month 4 = 22.5 [10.4]) from Month 0 to 4 (β = -7.24, p = 0.001, 95% confidence interval [CI] -11.39 to -3.09). The CFQ (mean [SD] baseline = 27.6 [8.0]; at Month 4 = 25.0 [8.0]) did not change significantly from Month 0 to 4 (β = -1.2, p = 0.482, 95% CI -4.5 to 2.1). Parallel mediation analyses indicated that decreases in the PCS and CFQ together (β = -6.1, SE = 2.5, 95% CI -11.6 to -1.8), and the PCS alone (β = -4.8, SE = 2.04, 95% CI -9.1 to -1.1), mediated changes in headache disability in the MBCT-M treatment completer group (n = 19). CONCLUSION: In this study, pain catastrophizing showed strong promise as a potential mechanism of MBCT-M. Future research should continue to explore cognitive appraisal changes in mindfulness-based interventions.