ObjectiveRoad traffic crash fatalities disproportionately affect young male drivers. Driver distraction is a leading contributor to crashes. Mind wandering (MW) is a prevalent form of driver distraction that is linked to certain unsafe driving behaviours that are associated with increased crash risk (e.g., faster driving). Negative mood can lead to MW, and thus may represent a causal pathway to MW-related unsafe driving. This preliminary pre-post (T1, T2), randomized, controlled, single-blinded experiment tested whether negative mood, compared to neutral mood, increases MW while driving as well as unsafe driving and emotional arousal during MW. It also tested the moderating contribution of trait rumination and inhibitory control to this proposed causal pathway. MethodsForty healthy male drivers aged 20 to 24 were randomly allocated to a negative or neutral mood manipulation involving deception. Individual differences in trait rumination and inhibitory control were measured at T1. At T1 and T2, participants drove in a driving simulator measuring driving speed, headway distance, steering behaviour, and overtaking. Heart rate and thought probes during simulation measured emotional arousal and MW, respectively. ResultsNegative mood exposure led to more MW while driving (Odds Ratio = 1.79, p = .022). Trait rumination positively moderated the relationship between negative mood and MW (Odds Ratio = 2.11, p = .002). Negative versus neutral mood exposure led to increases in headway variability (Cohen’s d = 1.46, p = .026) and steering reversals (Rate Ratio = 1.33, p = .032) during MW relative to focused driving. Between-group differences in emotional arousal were not significant. ConclusionResults support a causal pathway from negative mood to unsafe driving via MW, including the moderating contribution of trait rumination. If replicated, these preliminary findings may inform the development of interventions targeting this potential crash-risk pathway in vulnerable young driver subgroups.