Salt marshes often undergo rapid changes in lateral extent, the causes of which lack common explanation. We combine hydrological, sedimentological and climatological data with analysis of historical maps and photographs to show that long-term patterns of lateral marsh change can be explained by large-scale variation in sediment supply and its wave-driven transport. Over 150 years, northern marshes in Great Britain expanded while most southern marshes eroded. The cause for this pattern was a north to south reduction in sediment flux and fetch-driven wave sediment resuspension and transport. Our study provides long-term and large-scale evidence that sediment supply is a critical regulator of lateral marsh dynamics. Current global declines in sediment flux to the coast are likely to diminish the resilience of salt marshes and other sedimentary ecosystems to sea level rise. Managing sediment supply is not common-place, but may be critical to mitigating coastal impacts from climate change.