A 30 m discrepancy between sea-level records within the Gulf of Mexico and other “global” records has been known in the literature for over 50 years. Many hypotheses have been proposed to explain this discrepancy including sedimentary loading due to the Mississippi River and the effects of radiocarbon reservoirs. Using quantitative numerical models and over 50 new radiocarbon dates obtained from shallow marine systems across the Gulf of Mexico, we test several hypotheses that might explain this discrepancy. We found that glacio-hydro-isostasy, changes in the Earth’s shape due to the shifting of mass on the Earth’s surface caused by the waxing and wanning of the last great ice sheets, with an appropriate ice model for the Laurentide Ice sheet provides the best solution. Furthermore, this work has important implications for the ice sheet reconstructions for the Laurentide ice sheet and traditional sequence stratigraphic methods for correlating strata on passive continental margins.