Mass extinctions and biotic crises in the marine realm are often linked to major environmental perturbations, including large igneous provinces (LIPs) and asteroid impacts. We present a global synthesis of mercury (Hg) concentrations and isotope ratios from both marine and terrestrial archives across the Cretaceous/Paleogene (K/Pg) boundary to evaluate the environmental consequences of Deccan Traps volcanism. Data from 26 globally distributed sections and boreholes reveal log-normal distributions of Hg and widespread anomalies near the boundary, consistent with increased volcanic Hg emissions. Most Hg isotope values across the K/Pg boundary indicate a volcanic Hg source, based on the mass-independent (MIF) Δ¹⁹⁹Hg values. However, negative Δ¹⁹⁹Hg values suggest continental inputs in the Jhilmili section (India). We examined how global biotic changes responded to both Deccan volcanism and the Chicxulub impact. The evidence suggests that the end-Cretaceous mass extinction was a multifactorial event, primarily driven by volcanic-induced climate instability and ocean acidification, and likely intensified by the asteroid impact.