Clinical reasoning and decision-making in prehospital contexts are complex, and patient assessments may be influenced by stress or biases, thus potentially risking patient safety. Previous research has shown mixed results regarding cognitive interventions designed to counteract biases and improve decision-making. In educational settings, there are no tools that assess clinical reasoning while also measuring important decision-making outcomes. This study employed a mixed-methods design and a novel assessment model to evaluate clinical reasoning and decision-making among Swedish prehospital nurse specialists. Additionally, the effect of the metacognitive TWED mnemonic was investigated. Thirteen participants were randomly assigned to two groups and assessed patients in simulation settings, with groups switching cases after brief training on the TWED mnemonic. The primary outcomes included point-based scoring on decision-making, grading of potential risks of patient harm, and analysis of clinical reasoning through reflections. The results showed large variation, without overall differences between groups or demographics. A complex case presentation resulted in lower scores and greater risks of potential patient harm. Qualitative analysis highlighted participants’ ability to handle conflicting data, which correlated with better outcomes. The use of the TWED mnemonic may have increased commission bias. Further research is needed to validate and understand these findings.