Mindset, or the therapist’s series of beliefs which influence the actions and decisions taken in the course of their work, is considered fundamental in its impact on the effectiveness and length of a course of therapy. This article gives a definition of “mindset” and analyses its implications for psychotherapy in general and Single Session Therapy (SST) in particular. This is followed by an analysis of the literature surrounding SST, leading to the description of fourteen principles which make up the typical mindset of a single session therapist. The application of the principles is illustrated by a clinical case study.