Variations in Earth’s orbit pace global ice-volume and sea-level changes, but the variability in the response for different sectors of the Antarctic Ice Sheet to orbitally forced climate change remains unclear. Here we present geological records of iceberg-rafted debris and other proxies from locations adjacent to the West Antarctic Ice Sheet (WAIS) with comparisons to an existing East Antarctic Ice Sheet (EAIS) record over the time interval ~3.3–2.3 million years ago. Iceberg calving events from the WAIS recorded in Ross Sea sediment cores show a linear response to orbital forcing at timescales corresponding to obliquity (~40,000 years) and precession (~23,000–19,000 years) modulated by eccentricity (~100,000 years). This contrasts with an existing record adjacent to the EAIS, which does not contain obliquity pacing. Combined with ice-sheet model sensitivity tests, the geological data show that the WAIS is highly dynamic and responsive to oceanic melt driven by changes in Southern Ocean circulation, together with atmospheric forcing through variations in local insolation. Conversely, the EAIS appears less responsive to oceanic forcing, despite being the dominant source of meltwater to the global ocean during the mid-Pliocene. Our results imply a substantial role for atmospheric warming on mid-Pliocene sea-level from both WAIS and EAIS.