Animals can respond to dynamic environments through phenological plasticity of life history events, however, changes in one part of the annual cycle can diminish the success of subsequent life history events. Our aims were to determine the associations between reproduction and moult phenology across years and to quantify phenological plasticity across varying environmental conditions. We conducted demographic surveys of 4,252 flipper-tagged Weddell seals (Leptonychotes weddellii) in the Ross Sea, Antarctica during four austral summers. At each sighting, seals were assigned a moult code based on the visible presence of new fur, and the start date of each animal’s moult was back-calculated. This dataset contains data on pupping and moult timing for each animal used to address this question.