Geologic evidence and palaeoclimate simulations have indicated the existence of an extensive, interconnected megamonsoon system over the Pangaea supercontinent. However, the ways in which subsequent continental break-up about 180 million years ago and reassembly in the Cenozoic, as well as large global climatic fluctuations, influenced the transition to the modern global monsoon system are uncertain. Here we use a large set of simulations of global climate every 10 million years over the past 250 million years to show that the monsoon system evolved in three stages due to changes in palaeogeography: a spatially extensive land monsoon with weak precipitation in the Triassic period (>170 Ma), a smaller land monsoon with intense precipitation in the Cretaceous period (170–70 Ma) and a return to a broader, weaker monsoon in the Cenozoic era (<70 Ma). It is found that global-mean temperature variations have little impact on global land-monsoon area and intensity over tectonic timescales. Applying an analysis of atmospheric energetics, we show that these variations of the global land monsoon are governed by continental area, latitudinal location and fragmentation.