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Saltwater intrusion on coastal farmlands can render productive land unsuitable for agricultural activities. While the visible extent of salt-impacted land provides a useful saltwater intrusion proxy, it is challenging to identify in early stages. Moreover, associated ecological and economic impacts are often underestimated as reduced crop yields in farmlands surrounding salt patches are difficult to quantify. Here we develop a high-resolution (1 m) dataset showing salt patches on farm fringes and quantify the extent of salt-impacted lands across the Delmarva Peninsula, United States. Our method is transferable to other regions across and beyond the mid-Atlantic with similar saltwater intrusion issues, such as Georgia and the Carolinas. Our results show that between 2011 and 2017, visible salt patches almost doubled and 8,096 ha of farmlands converted to marsh—another saltwater intrusion consequence. Field-based electrical conductivity measurements show elevated salinity values hundreds of metres from visible salt patches, indicating the broader extent of at-risk farmlands. More farmland areas were within 200 m of a visible salt patch in 2017 compared to 2011, a rise ranging between 68% in Delaware and 93% in Maryland. On the basis of assumed 100% profit loss in at-risk farmlands within a 200 m buffer around salt patches in 2016–2017, the range of economic losses was estimated between US$39.4 million and US$107.5 million annually, under 100% soy or corn counterfactuals, respectively.