Mountain building exposes fossil organic carbon (OCfossil) in exhumed sedimentary rocks. Oxidation of this material releases carbon dioxide from long-term geological storage to the atmosphere. OCfossil is mobilized on hillslopes by mass wasting and transferred to the particulate load of rivers. In large fluvial systems, it is thought to be oxidised in transit, but in short, steep rivers that drain mountain islands, OCfossil may escape oxidation and re-enter geological storage due to rapid fluvial transfer to the ocean. In these settings, the rates of OCfossil transfer and their controls remain poorly constrained. Here we quantify the erosion of OCfossil from the Taiwan mountain belt, combining discharge statistics with measurements of particulate organic carbon load and source in 11 rivers. Annual OCfossil yields in Taiwan vary from 12 ± 1 to 246 ± 22 tC km−2 yr−1, controlled by the high physical erosion rates that accompany rapid crustal shortening and frequent typhoon impacts. Efficient transfer of this material ensures that 1.3 ± 0.1 × 106 tC yr−1 of OCfossil exhumed in Taiwan is delivered to the ocean, with <15% loss due to weathering in transit. Our findings suggest that erosion of coastal mountain ranges can force efficient transfer and long-term re-accumulation of OCfossil in marine sediments, further enhancing the role of mountain building in the long-term storage of carbon in the lithosphere.