Swedish underground mines are constantly improving their safety. However, major incidents still occur, and the extreme environment poses challenges during rescue operations. The aim of this study was to evaluate prehospital medical management during a full-scale exercise in an underground mine in order to gain knowledge on the leadership and decision-making of the medical incident commander. We used a qualitative single-case study design following a full-scale exercise that included emergency medical services (EMSs), rescue services, and a mining company. The exercise was documented through on-site observation notes, audio recordings, and video recordings, all of which were written out as text and analysed using qualitative content analysis. The results showed that few decisions were made, and without all available medical information, and that they were made by others than the medical incident commander. This resulted in a delay in decision-making, in vital treatment, and in transport of patients from the site. Clearer leadership and more active decisions might have resulted in a different outcome for the injured parties.