The early Aptian recorded one of the most significant episodes of environmental change during the Mesozoic—the Selli oceanic anoxic episode (OAE 1a). It has often been suggested that magmatic activity related to the emplacement of the Greater Ontong Java large igneous province (LIP) triggered OAE 1a. A major challenge, however, resides in the establishment of precise temporal relationships between the environmental perturbations associated with OAE 1a and the phases of volcanic activity. In this study we evaluate the potential of mercury (Hg) as a proxy of volcanic activity and investigate lower Aptian sediments with different total organic carbon (TOC) contents, which are exposed at Roter Sattel (Briançonnais, Swiss Alps), Glaise (Vocontian Basin, southeast France), and La Bédoule (South Provencal Basin, southeast France). The intervals equivalent to OAE 1a are marked by significant increases in Hg contents, which are only partially dependent on TOC contents. This is shown by the Hg anomalies in the TOC-poor sediments of La Bédoule, the only moderate correlation of Hg and TOC contents in the TOC-enriched sediments of Roter Sattel (R2 = 0.48), and the persistence of the anomaly in Hg/TOC ratios in all sediments except for the TOC-enriched ones. These results suggest that the Hg anomaly not only is related to primary productivity, redox conditions, and organic-matter preservation, but has deeper roots. Volcanic outgassing related to Greater Ontong Java LIP activity is taken here as the main source of the Hg enrichment recorded in the western Tethyan sediments. Our Hg data indicate that magmatic pulses at the onset and during the OAE 1a triggered the early Aptian environmental perturbations.