Accurately reconstructing the evolution of the Asian monsoon is predicated on understanding the impact of temperature, precipitation, and tectonic paleogeography on silicate weathering proxies over million year timescales. We find that decreasing trends in chemical weathering proxies in both the northern and southern Tibetan Plateau match those from benthic oxygen isotope and sea surface temperature stacks since the late Miocene. In contrast, a synthesis of magnetic parameter-based records and marine clastic flux records reveals stronger monsoon precipitation during both the warm middle Miocene and cool late Miocene, supporting model simulations showing that both paleogeography and atmospheric CO2 content are important in controlling the summer monsoon. This trend of increasing monsoon precipitation contrasts with the record of decreasing chemical weathering, suggesting that chemical weathering proxies are mainly regulated by temperature at million year timescales, rather than precipitation. These findings clarify reconstructions of the Cenozoic evolution of the East Asian summer monsoon. Plain Language Summary Understanding the Cenozoic history of the Asian summer monsoon is important for identifying its forcing mechanisms. Both magnetic and geochemical parameters have been used to reconstruct the Asian monsoon, but they show conflicting trends over the past 5 million years (Myr). Here we extend the geochemical record of the East Asian summer monsoon back to 20 Myr ago and compare it with variations in magnetic parameters and global temperature. The comparison reveals that, on million year timescales, chemical weathering proxies are mainly controlled by temperature. Based on a synthesis of available marine and terrestrial monsoon precipitation records, we conclude that the East Asian summer monsoon intensified during high CO2 conditions 17-14 Myr ago, weakened 14-11 Myr ago due to climate cooling, but intensified again at 11-7 and 4-2.7 Myr ago, driven by tectonic changes in paleogeography such as Tibetan plateau uplift and closure of the Panama Seaway. This reconstruction supports model simulations that Cenozoic paleogeographic variations played an important role in controlling the long-term evolution of the Asian monsoon.