Introduction: Workplace-based assessment is integral to programmatic assessment in a competency-based curriculum. In 2013, one such assessment, a mini-Clinical Evaluation Exercise (mini-CEX) with a novel “entrustability scale”, became compulsory for over 1,200 Australia and New Zealand College of Anaesthetists (ANZCA) trainees. We explored trainees’ and supervisors’ understanding of the mini-CEX, their experience with the assessment, and their perceptions of its influence on learning and supervision. Methods: We conducted semi-structured telephone interviews with anesthesia supervisors and trainees and performed an inductive thematic analysis of the verbatim transcripts. Results: Eighteen supervisors and 17 trainees participated (n = 35). Interrelated themes concerned the perceived purpose of the mini-CEX, its value in trainee learning and supervision, and the process of performing the assessment. While few participants saw the mini-CEX primarily as an administrative burden, most focused on its potential for facilitating trainee improvement and reported positive impacts on the quantity and quality of feedback, trainee learning, and supervision. Finding time to schedule assessments and deliver timely feedback proved to be difficult in busy clinical workplaces. Views on case selection were divided and driven by contrasting goals – i.e., receiving useful feedback on challenging cases or receiving a high score by choosing lenient assessors or easy cases. Whether individual mini-CEXs were summative or formative was subject to intense debate, while the intended summative use of multiple mini-CEXs in programmatic assessment was poorly understood. Conclusion: Greater clarity of purpose and consistency of time commitment are necessary to embed the mini-CEX in the culture of the workplace, to realize the full potential for trainee learning, and to reach decisions on trainee progression.