Objectives Overwork has been recognized as a risk factor for cerebrovascular and cardiovascular disease (CCVD). To our best knowledge, Japan is the first country in the world to implement an independent act (the 2014 Act) for the prevention of overwork‐related disease, which was promoted through application of preventive measures, such as reductions in working hours. We assessed changes in working hours and overwork‐related CCVD before and after introduction of the 2014 Act. Methods We calculated the overwork‐related CCVD incidence rate and average monthly working hours for 10 industry groups in Japan with data from 2012 to 2018. We applied a causal mediation analysis to estimate the total effect of implementing the 2014 Act on the overwork‐related CCVD and the effect mediated by working hours. Results An average of 271 for every 48 million employees developed overwork‐related CCVD per year. After introducing the 2014 Act, the incidence rate ratio of overwork‐related CCVD was 0.881‐fold lower (95% CI 0.780‐0.995) compared with before the policy change. The 2014 Act contributed to a decrease of 26% (78 cases per year; 95% CI 29‐173) of the overwork‐related CCVD incidence per year. Approximately 41% (32 cases per year) of this effect could be explained by reduced working hours. Conclusions Our study highlights the impact of the 2014 Act in Japan on the reduction in working hours, which further contributes to the reduction in overwork‐related CCVD. Policymakers should consider adopting our innovative approach to assess the mediation effect underlying the implementation of new policies.