The local warming occurring within the western Antarctic Peninsula (WAP) is causing some of the greatest environmental shifts on the planet. Over the past 140 years there have been profound biological and physical perturbations to the wildlife on the WAP. We examined whether trophic level shifts were evident in the top predators within this system and use stable isotope signatures as a proxy. Contemporary WAP leopard seals have different nitrogen isotopic values to other leopard seal populations, however historically WAP leopard seals had δ15N values within the range of other western and eastern Antarctic populations. From our 140 yr record of leopard seal tissues (n = 167), we show that the δ15N values of WAP leopard seal tissues have dropped significantly, and that this change occurs as a step around the 1980s. The magnitude (2.6o/oo δ15N) of change is ecologically significant as it reflects a drop of a trophic level within the contemporary WAP food web. Values suggest that leopard seals have shifted from eating vertebrates to krill. Over this same time period there was no shift in δ15N of the WAP krill-feeding specialist, the crabeater seal, which supports the idea that the change in leopard seal δ15N values is not due to a baseline shift in the nitrogen isotope values. Despite the century-long perturbation in the WAP, the top-predators, the seals, show a trophic downshift only in recent times, post 1980s.