The increasing use of direct observation tools to assess routine performance has resulted in the growing reliance on assessor-based judgements in the workplace. However, we have a limited understanding of how assessors make judgements and formulate ratings in real world contexts. The current research on assessor cognition has largely focused on the cognitive domain but the contextual factors are equally important, and both are closely interconnected. This study aimed to explore the perceived cognitive and contextual factors influencing Mini-CEX assessor judgements in the Emergency Department setting. We used a conceptual framework of assessor-based judgement to develop a sequential mixed methods study. We analysed and integrated survey and focus group results to illustrate self-reported cognitive and contextual factors influencing assessor judgements. We used situated cognition theory as a sensitizing lens to explore the interactions between people and their environment. The major factors highlighted through our mixed methods study were: clarity of the assessment, reliance on and variable approach to overall impression (gestalt), role tension especially when giving constructive feedback, prior knowledge of the trainee and case complexity. We identified prevailing tensions between participants (assessors and trainees), interactions (assessment and feedback) and setting. The two practical implications of our research are the need to broaden assessor training to incorporate both cognitive and contextual domains, and the need to develop a more holistic understanding of assessor-based judgements in real world contexts to better inform future research and development in workplace-based assessments.