OBJECTIVE: Driver distraction contributes to fatal and injury crashes in young drivers. Mind wandering (MW) is a covert form of distraction involving task-unrelated thoughts. Brief online mindfulness training (MT) may reduce unsafe driving by enhancing recognition (meta-awareness) of MW and reducing its occurrence. This pilot trial tested these proposed mechanisms of MT and explored its specificity of action, effects on driving behaviour in simulation, as well as intervention adherence and acceptability in young drivers. METHODS: A pre-post (T1, T2), randomized, active placebo-controlled, double-blinded design was used. Twenty-six drivers, aged 21-25, received either brief online MT (experimental) or progressive muscle relaxation (PMR, control) over 4-6 days. A custom website blindly conducted randomization, delivered interventions, administered questionnaires, and tracked adherence. At T1 and T2, a simulator measured driving behaviour while participants indicated MW whenever they recognized it, to assess meta-awareness, and when prompted by a thought-probe, to assess overall MW. RESULTS: MT reduced MW while driving in simulation. The MT group reported higher state mindfulness following sessions. Motivation did not account for MW or mindfulness results. MT and meta-awareness were associated with more focus-related steering behaviour. Intervention groups did not significantly differ in adherence or attrition. No severe adverse effects were reported, but MT participants reported more difficulty following intervention instructions. CONCLUSION: Results support a plausible mechanism of MT for reducing MW-related crash risk (i.e., reduction of MW) in young drivers. This preliminary evidence, alongside promising online adherence and acceptability results, warrants definitive efficacy and effectiveness trials of online MT.