Background: In Danish GP training we had the ambition to enhance and assess global reflection ability, but since we found no appropriate validated method in the literature, we decided to develop a new assessment tool. This tool is based on individual trainee developed mind maps and structured trainer-trainee discussions related to specific complex competencies. We named the tool Global Assessment of Reflection ability (GAR) and conducted a mixed method validation study. Our goal was to investigate whether it is possible to enhance and assess reflection ability using the tool. Methods: In order to investigate acceptability, feasibility, face validity, and construct validity of the tool we conducted a mixed method validation study that combined 1) qualitative data obtained from 750 GP trainers participating in train-the-trainer courses, 2) a questionnaire survey sent to 349 GP trainers and 214 GP trainees and 3) a thorough analysis of eight trainer-trainee discussions. Results: Our study showed an immediate high acceptance of the GAR tool. Both trainers and trainees found the tool feasible, useful, and relevant with acceptable face validity. Rating of eight audio recordings showed that the tool can demonstrate reflection during assessment of complex competencies. Conclusions: We have developed an assessment tool (GAR) to enhance and assess reflection. GAR was found to be acceptable, feasible, relevant and with good face- and construct validity. GAR seems to be able to enhance the trainees' ability to reflect and provide a good basis for assessment in relation to complex competencies.