Using a novel hand-collected dataset of mandatory greenhouse gas (GHG) disclosure regulations across 45 countries from 2000 to 2023, this study examines whether disclosure mandates reduce corporate emissions. Employing a staggered difference-in-differences design with 45,547 firm-year observations, we find that mandatory GHG disclosure leads to a 7.5% reduction in emission intensity. We further investigate how regulatory design features and institutional factors shape disclosure effectiveness. Our results indicate that disclosure mandates tend to be more effective when issued by governments rather than stock exchanges, when structured as mandatory requirements rather than “comply-or-explain” regimes, when specifying clear disclosure scopes, when focusing specifically on GHG emissions rather than broad ESG topics, and when targeting specific industries rather than the entire economy. We also find that formal institutions (carbon pricing mechanisms, climate policy uncertainty, and legal origin) and informal institutions (national climate performance and cultural individualism) significantly condition the effectiveness of mandatory disclosure. Further analysis reveals that mandatory disclosure reduces not only Scope 1 and 2 emissions but also Scope 3 emissions, suggesting genuine environmental improvement rather than carbon offshoring to supply chains. Our findings contribute to the debate on the effectiveness of disclosure as a climate policy tool and offer insights for policymakers designing climate disclosure frameworks.