Causal connections among major Ordovician environmental and biological  events (i.e., long-term climatic cooling, Hirnantian Glaciation, Great Ordovician Biodiversification Event, and Late Ordovician Mass Extinction) remain in  debate, and the hypothesis of volcanism-driven long-term cooling is untested.  Here, we present both newly generated and compiled literature data for global  volcanic activity (Hg geochemistry), sea-surface temperatures (conodont  oxygen isotopes), and chemical weathering intensity (conodont strontium  isotopes). This dataset documents a ~25-Myr-long interval of climatic cooling  (~470-445 Ma), commencing around the onset of the Middle Ordovician,  intensifying during the Late Ordovician, and ultimately culminating in the  Hirnantian Glaciation. Cooling was associated with long-term intensified volcanic rock (basalt) weathering and atmospheric pCO2 drawdown, as well as  episodic marine photic-zone euxinia, during and after major volcanic episodes. These relationships favor volcanism (i.e., continental large igneous  provinces) as the primary driver of contemporaneous environmental and climatic changes, thus revealing complex modulation of life-environment coevolution during the Ordovician Period.