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Abstract The Cambro‐Ordovician interval marks a significant transition from extinction to bio‐diversification in deep time. However, the relationship of bio‐transition to volcanism, commonly characterized by mercury (Hg) systematics in sedimentary records, has not been examined. We present the first Cambro‐Ordovician Hg systematics from the Scandinavian Alum Shale. Our results show pronounced Furongian Hg enrichments, coupled with positive Δ¹⁹⁹Hg, Δ²⁰⁰Hg, and Δ²⁰¹Hg values and negative Δ²⁰⁴Hg values that we ascribe to atmospheric Hg transport over long‐distances, while Early Ordovician Hg anomalies, characterized by near‐zero mass‐independent isotope values, indicative of submarine source. Our findings are supported by two new proxies: molybdenum‐Hg and vanadium‐δ²⁰²Hg co‐variations, demonstrating Hg systematics were strongly influenced by changes in source and depositional conditions. Constrained by a synchronous atmospheric‐tectonic‐oceanic model, we hypothesize Furongian subaerial volcanism contributed to global extinction and oceanic anoxia, whereas Early Ordovician submarine volcanism concurrent with ocean water upwelling promoted the nascent bio‐diversification.