The Hirnantian Stage of the Late Ordovician coincides with a positive carbon isotope excursion (HICE, ~+6‰), a major glaciation, increased volcanic activity, expanded marine anoxia, and one of the largest mass extinctions of the Phanerozoic. The origin of the HICE is debated, with proposed mechanisms favoring enhanced low-latitude carbonate weathering and/or increased efficiency of organic carbon burial. To test those hypotheses, we assembled new and published δ¹³C_carb and δ¹³C_org data and shale phosphorus and carbonate-associated phosphate concentrations from diverse depositional settings on several continents. We then evaluated these results using an integrated carbon cycle model derived from GEOCARB, and a coupled oceanic carbon and phosphorus cycle model. This approach yielded quantitative tests of the response of the global carbon cycle to changes in volcanic degassing, silicate and carbonate weathering rates, and organic carbon burial, individually and in combination, to determine which forcings yield signals most aligned with observed δ¹³C_carb and δ¹³C_org records. On this basis, the most plausible scenario for the HICE involves a ~50% decrease in weathering of silicates and organic carbon at higher latitudes combined with enhanced carbonate weathering (f_wcarb = 88%) at lower latitudes. Synchronous, globally enhanced burial of organic carbon was due to more efficient nutrient cycling caused by stronger thermohaline circulation and increased volcanism that further enriched oceanic dissolved inorganic carbon (DIC) in ¹³C. The influence of increased input of ¹³C-depleted carbon from volcanism and metamorphism may have been overwhelmed by these drivers of the positive δ¹³C shift. The Hirnantian Glaciation was likely initiated by secular enhancement of continental weathering due to the expansion of early plants but was terminated by increased volcanism and decreased CO₂ consumption by silicate weathering during the cooling. This scenario is consistent with the timeline of known geological events and offers insights into the mechanisms responsible for the associated biotic crisis. Specifically, the onset of the Hirnantian Glaciation induced lower global temperatures, resulting in sea-level fall and a loss of habitat space, while its termination led to enhanced organic carbon export and deep-water anoxia, all of which likely contributed to the Late Ordovician mass extinction.