OBJECTIVE: It is often assumed that psychosocial pain treatments work because of specific active components of the intervention. The degree to which common factors may contribute to improved pain outcomes is not well researched. The purpose of this study was to examine patient-related and therapist-related common factors during a Mindfulness-Based Cognitive Therapy (MBCT) for headache pain trial. MATERIALS AND METHODS: This study was a secondary analysis of a parallel group, unblinded, randomized controlled trial in which MBCT was compared with a control. A series of linear regression models and 1 bootstrap mediation model were conducted with the sample of participants that completed MBCT (N=21). RESULTS: In-session participant engagement was positively associated with treatment dose indicators of session attendance (P=0.038) and at-home meditation practice (P=0.027). Therapist adherence and quality were both significant predictors of posttreatment client satisfaction (P=0.038 and 0.034, respectively). Therapist appropriateness was not significantly associated with any of the variables of interest (P>0.05). Baseline pain intensity was positively associated with pretreatment expectations and motivations (P=0.049) and working alliance (P=0.048), and working alliance significantly predicted posttreatment patient satisfaction (P<0.001). Higher pretreatment expectations and motivation significantly predicted greater improvement in pretreatment to posttreatment change in pain interference (P=0.016); however, this relation was fully mediated by baseline pain intensity (P<0.05). DISCUSSION: Common factors play an important role in improving pain outcomes and patient satisfaction during a MBCT for headache pain intervention. Stimulating positive pretreatment expectations and patient motivation, as well as building strong rapport is an important component of treatment success.