Fjord sediments are recognized as hotspots for the burial and storage of organic carbon, yet little is known about the long-term drivers of significant terrestrial organic carbon (OC) transfers into these coastal carbon stores. The mid-latitude fjord catchments of Scotland have a long history of human occupation and environmental disturbance. We provide new evidence to show that increased anthropogenic disturbances over the last 500 years appear to have driven a step change in the magnitude of terrestrial OC transported to the coastal ocean. Increased pressures from mining, agriculture and forestry over the latter half of the last millennium have destabilized catchment soils and remobilized deep stores of aged OC from the catchment to the coastal ocean. Here we show that fjord sediments are capable of acting as highly responsive and effective terrestrial OC sinks, with OC accumulation rates increasing up to 20% during the peak period of anthropogenic disturbance. The responsiveness and magnitude of the fjord OC sink represents a potentially significant time-evolving component of the global carbon cycle that is currently not recognized but has the potential to become increasingly important in the understanding of the role of these coastal carbon stores in our climate system.