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Abstract Until recently, information about the end of the Cretaceous was based upon investigation of global outcrop sections. New subsurface drilling and characterization from well cores and logs in the Gulf of Mexico Basin have greatly illuminated the end Cretaceous event. However, the palaeogeography of the late Maastrichtian just prior to bolide impact is less well understood and is of great importance in terms of modelling the resulting distribution and composition of the Chicxulub impact material, as well as tsunami and seiche wave height. Here, we examine the Maastrichtian strata in the basin, synthesizing lithostratigraphy and chronostratigraphy, tectonic plate reconstructions, global and local sea level history, palaeoclimate and depositional systems. Our new Maastrichtian palaeogeographical reconstruction shows the basin prior to the Chicxulub impact at a time of globally high sea level, with widespread deposition of deepwater chalks and shallow marine carbonates and local siliciclastic shorelines fed by the nascent Cordilleran belt. Stratigraphic correlations of wells and outcrops illustrate the range of palaeoenvironments from coastal plain to deep marine. As much as 610 m (2000 ft) of Maastrichtian and Campanian section is mapped around the basin, reflecting accommodation provided by basin subsidence, salt deflation and palaeophysiography. A large thickness of carbonates accumulated in the basin centre, with steep shoreline to basin gradients particularly in Mexico. At the end of the Cretaceous, carbonate palaeoenvironments probably covered 96% of the Gulf of Mexico Basin, with less than 4% of the area likely occupied by siliciclastic systems, a distribution that evolved from the Early Cretaceous. Our maps thus explain dominance of carbonate breccia and chalks in K–Pg boundary units deposited over the basin sites proximal or distal to the Chicxulub impact crater. This also elucidates the large impedance contrast and high amplitude seismic response of the K–Pg boundary horizon, mappable over vast portions of the basin.