Introduction: Effective trauma resuscitation requires the coordinated efforts of an interdisciplinary team. Mental practice (MP) is defined as the mental rehearsal of activity in the absence of gross muscular movements and has been demonstrated to enhance acquiring technical and procedural skills. The role of MP to promote nontechnical, team-based skills for trauma has yet to be investigated. Methods: We randomized anaesthesiology, emergency medicine, and surgery residents to two-member teams randomly assigned to either an MP or control group. The MP group engaged in 20 minutes of MP, and the control group received 20 minutes of Advanced Trauma Life Support (ATLS) training. All teams then participated in a high-fidelity simulated adult trauma resuscitation and received debriefing on communication, leadership, and teamwork. Two blinded raters independently scored video recordings of the simulated resuscitations using the Mayo High Performance Teamwork Scale (MHPTS), a validated team-based behavioural rating scale. The Mann-Whitney U-test was used to assess for between-group differences. Results: Seventy-eight residents provided informed written consent and were recruited. The MP group outperformed the control group with significant effect on teamwork behaviour as assessed using the MHPTS: r = 0.67, p < 0.01. Conclusions: MP leads to improvement in team-based skills compared to traditional simulation-based trauma instruction. We feel that MP may be a useful and inexpensive tool for improving nontechnical skills instruction effectiveness for team-based trauma care.