Background Although workplace mental health screening is often implemented to aid early identification of mental health symptoms and facilitate access to treatment, supporting evidence is limited. Aims We aimed to evaluate the effect of independently conducted, confidential, online mental health screening, paired with automated tailored feedback recommending referral services, on help-seeking and psychological distress. Method We conducted a cluster-randomised controlled trial with firefighters from an Australian fire and rescue service. Randomisation occurred by station (N = 264). Firefighters at stations allocated to the intervention group received tailored information detailing suitable mental health services based on their Kessler-6 psychological distress score (K6). The control group received generic feedback on services irrespective of K6 score. The primary outcome was help-seeking at 3-months post-intervention for those with at least moderate levels of psychological distress at baseline (K6 ≥14). The study was registered with Australian New Zealand Clinical Trials Registry (no. ANZCTR 12621001457831). Results Of the 459 firefighters screened, 141 (30.72%) scored ≥14 on K6. Among this subgroup at 3 months, no differences were observed in rates of overall help-seeking between the intervention and control groups (P = 0.31). In contrast, levels of psychological distress remained high in the intervention group but declined in the control group (t[111] = 2.29, 95% CI: 0.24, 3.23, P = 0.024). The difference in psychological distress associated with workplace mental health screening equated to an effect size of −0.42 (95% CI: −0.04, −0.79). Conclusions Our findings suggest that independent, confidential online mental health screening, paired with tailored online feedback and information on available treatment, does not significantly increase help-seeking and may sustain psychological distress over time compared with receiving generic information. As such, it should not be implemented to promote help-seeking and reduce levels of psychological distress. These findings are relevant for workplaces, mental health researchers and practitioners alike, highlighting the potential risk and potential harm of mental health screening conducted in this way on individuals.