Understanding human impacts on ecosystems is critical for conservation, but can be complicated by interactions between multiple impacts occurring at different times. Using historical patterns of ditch construction on Cape Cod, Massachusetts, we tested the hypothesis that mosquito ditches have exacerbated salt marsh die-offs. Ditching activities occurred in the 1930s and were followed by post-World War II shoreline development, which created > 90% of current shoreline infrastructure on Cape Cod. Recently, predator depletion caused by recreational fishing has allowed populations of a native herbivorous crab (Sesarma reticulatum) to increase dramatically, triggering herbivore-driven cordgrass (Spartina alterniflora) die-off at developed sites. Depression-era mosquito ditching had little effect for decades, but accelerated subsequent die-offs by expanding cordgrass habitat. Despite occurring decades apart, ditching interacted synergistically with shoreline development and recreational fishing to devastate ~ 55% of low marsh habitat (the narrow band of marsh grass necessary for marsh persistence and expansion). This suggests that historical human impacts can remain dormant for decades before interacting unexpectedly with modern perturbations. Such latent impacts are widespread in both marine and terrestrial habitats and may be common in other ecosystems with a history of disturbance.