The process-based approach assumes that transdiagnostic psychological processes are involved in the onset and maintenance of mental disorders. Case conceptualization is used to identify such psychological processes and to individualize the intervention accordingly. This approach is fundamentally different from syndrome-based approaches in which standardized treatments are administered for psychiatric diagnoses or symptoms. In the current case, we proposed a process-based conceptualization and treatment for a woman with concomitant problematic substance use and emotional symptoms. Our idiosyncratic process-based conceptualization showed that for this person, substance abuse consisted in a maladaptive coping strategy to deal with repetitive negative thinking and poor emotion regulation skills, for which we decided to use a mindfulness-based intervention. Treatment comprised 8 weeks of individually delivered mindfulness-based cognitive therapy sessions. A comprehensive assessment of psychiatric symptoms and psychological processes was conducted at baseline, post-intervention and 3-month follow-up. The post-treatment assessment revealed that psychological processes targeted by the intervention were improved, together with the mindfulness trait. At the symptom-based level, we observed a significant reduction in substance abuse and emotional symptoms. Treatment gains were maintained at follow-up. From a process-based perspective, this result implies that the intervention successfully targeted the psychological processes underlying the presenting symptoms. Adopting a process-based approach rather than symptom- or syndrome-based approaches is a valuable alternative in the conceptualization and treatment of complex cases presenting with multiple comorbidities.