Objective: Mindfulness-Based Cognitive Therapy (MBCT) is a promising psychosocial intervention for adult ADHD. The feasibility and effectiveness of an adapted MBCT program is explored, together with the possible process of change. Method: Mixed-method study with 31 ADHD patients participating in an adapted MBCT program. Self-report questionnaires on ADHD symptoms, executive functioning, mindfulness skills, self-compassion, patient functioning, and health status were administered before and after MBCT. Semi-structured interviews were conducted with 24 patients. Results: A modest drop-out of n = 5 (16%) was found. MBCT resulted in a significant reduction of ADHD symptoms and improvements of executive functioning, self-compassion, and mental health. Qualitative analysis provided insight in facilitators and barriers participants experienced, and their process of change. Conclusion: The adapted MBCT program seemed to be feasible for adults with ADHD and preliminary evidence for the effectiveness is shown. An adequately powered Randomized Controlled Trial (RCT) is needed to further examine the effectiveness of MBCT for ADHD.