Multimethod approaches are a common form of methodology employed in social sciences, however are less commonly used in the examination of clinical interventions for mental health conditions. This case study outlines the strengths and challenges of applying a multimethod approach to the examination of a brief intervention for personality disorder within a stepped model of care. The multimethod approach chosen here combines different research designs to provide a richer understanding of an issue. Personality disorders are complex and represent about a quarter of all mental health emergency and hospital patients, meaning studying the clinical pathways of care and trajectories through the health care system is a priority both in terms of service delivery and cost-benefit considerations. By using a combination of service-use data and pre-post survey data, we were able to evaluate the efficacy of the brief intervention in terms of symptom change, retention within the intervention, and pathways of care within the health service during and following the intervention. The importance of collaboration, data handling, organization, and ethical considerations is discussed. Overall, the multimethod approach has several strengths, which could be used as a framework to evaluate other real-world clinical interventions being used within busy mental health services.