<p>Marine anoxia is widely proposed as a kill mechanism for the Devonian-Carboniferous boundary mass extinction commonly linked to the end-Devonian Hangenberg Crisis (~359.3 Ma), but the exact connections between primary drivers, marine redox conditions, and biotic turnover are not well constrained. Mercury concentrations and isotopes from multiple sections in South China and western Canada show no significant signature of enhanced volcanism during the end-Devonian Hangenberg Crisis. Rather, the distinct negative shifts of Δ¹⁹⁹Hg coupled with positive shifts of δ²⁰²Hg across the end-Devonian Hangenberg Crisis provide compelling evidence for development of photic zone euxinia, as supported by covariant pyrite framboid data and Ce/Ce* values of carbonate. The photic zone euxinia was likely driven by increased nutrient delivery given its coincidence with carbon isotopic perturbations and enhanced weathering immediately before the end-Devonian Hangenberg Crisis. Our data highlight the possibility that photic zone euxinia played a pivotal role in the extinction of marine biota during the end-Devonian Hangenberg Crisis by limiting the habitable space and by increased hydrogen sulfide toxicity, especially for organisms living in shallow water.</p>