Mixed reality offers an innovative approach to training complex medical skills in mass casualty incident simulations. This pre-post test study aimed to describe mixed reality feature priorities for mass casualty incident simulation from the end-users’ perspective and to explore the extent to which priorities changed following implementation. A mixed reality training program was developed to improve medical first responders’ triage skills in mass casualty incidents. Five European emergency medical service end-user organizations were asked to prioritize mixed reality training features in five content areas: the live training system, equipment, debriefing dashboard, analytics and statistics, and the manikin. The baseline (T1) survey was completed in late-2022. Six field trials were carried out between June 2023 and January 2024. The follow-up (T2) survey and group discussions were completed in early-2024. Follow-up responses tended to be more polarizing, with fewer features rated as must have (T1, 54.1%; T2, 46.4%), and more features rated as should have (T1, 19.6%; T2, 23.9%), could have (T1, 17.6%; T2, 18.4%), and won’t have (T1, 6.4%; T2, 10.7%). Prior to implementation, end-user organizations prioritized mixed reality simulation features that promoted realism. Following implementation end-users were more pragmatic, assigning higher ratings to features that aligned with the required learning goals and objectives for mixed reality mass casualty incident triage training. Changes in end-users’ priorities demonstrate a need to evaluate mixed reality simulation feature priorities before, during, and after implementation to ensure relevant feature selection and technology development.