Background: Workplace-based assessment (WBA) is critical to graduating competent physicians. Developing assessment tools that combine the needs of faculty, trainees, and governing bodies is challenging but imperative. Entrustable professional activities (EPAs) are emerging as a clinically oriented framework for trainee assessment. Objective: We sought to develop an EPA-based WBA tool for pediatric critical care medicine (PCCM) fellows. The goals of the tool were to promote learning through benchmarking and tracking entrustment. Methods: A single PCCM EPA was iteratively subdivided into observable practice activities (OPAs) based on national and local data. Using a mixed-methods approach following van der Vleuten's conceptual model for assessment tool utility and Messick's unified validity framework, we sought validity evidence for acceptability, content, internal structure, relation to other variables, response process, and consequences. Results: Evidence was gathered after 1 year of use. Items for assessment were based on correlation between the number of times each item was assessed and the frequency professional activity occurred. Phi-coefficient reliability was 0.65. Narrative comments demonstrated all factors influencing trust, identified by current literature, were cited when determining level of entrustment granted. Mean entrustment levels increased significantly between fellow training years (P = .001). Compliance for once- and twice-weekly tool completion was 50% and 100%, respectively. Average time spent completing the assessment was less than 5 minutes. Conclusions: Using an EPA-OPA framework, we demonstrated utility and validity evidence supporting the tool's outcomes. In addition, narrative comments about entrustment decisions provide important insights for the training program to improve individual fellow advancement toward autonomy.