Our primary objective was to understand the barriers and facilitators associated with the implementation of high-quality clinical practice guidelines (CPGs) for depression and anxiety in patients with dementia or Parkinson’s disease (PD). We conducted focus groups or interviews with participants experiencing dementia or PD, their caregivers, and physicians in Calgary, Alberta, and applied the theoretical domains framework and behaviour change wheel to guide data collection and perform a framework analysis. Thirty-three physicians and seven PD patients/caregivers participated. We report barriers and facilitators to the implementation of guideline recommendations for diagnosis, management, and the use of the guidelines. An overarching theme was the lack of evidence for depression or anxiety disorders in dementia or PD, which was prominent for anxiety versus depression. Patients noted difficulties with communicating symptoms and accessing services. Although guidelines are available, physicians have difficulty implementing certain recommendations due primarily to a lack of evidence regarding efficacy.