East Asian summer monsoon precipitation has a globally unique δ18O signature characterized by low δ18O values related to the precipitation “amount effect”. We explore the history of this signature using carbonate clumped-isotope thermometry and δ18O of paleosol carbonates from northern China. We find that soil water δ18O throughout the late Miocene and Pliocene was indistinguishable from present-day summer meteoric water δ18O. Additionally, soil temperatures were similar to present-day summer temperatures, as were latitudinal gradients in temperature. Paleosol carbonate δ13C values document a pattern of northward-increasing C4 vegetation during 7–3 Ma, interpreted as marking a paleobiome transition from forest to steppe. The present-day summer monsoons give rise to a similar transition owing to latitudinal gradients in precipitation amount and seasonal duration. Taken together, these lines of evidence point to active East Asian summer monsoons during the warm climates of the late Miocene and Pliocene.