The goal of this study was to evaluate the impact of team member familiarity on both technical and non-technical skills of police officers and to examine the mediating role of transactive memory components (specialization, coordination and credibility). Student police officers (N = 150) carried out professional simulation sessions in groups of three, with either familiar (n = 22 teams) or unfamiliar (n = 28 teams) team members, followed by a skills evaluation exercise. Professional instructors evaluated skills through direct observation of team member behaviours. The results revealed that training in familiar teams improved police officers’ technical and non-technical skills and transactive memory system. The effect of team familiarity on skills was mediated by transactive memory and specifically by the tacit coordination component. The findings are discussed in relation to the literature on the effects of team member familiarity on team performance, and implications for team training and staffing (turnover of personnel) in actions teams.Practitioner Summary: This study examined the effect of team member familiarity on both the technical and non-technical skills of police officers. Training in familiar teams improved skills and transactive memory. The positive effect of team member familiarity on skills was mediated by the coordination facet of transactive memory.