The Late Ordovician mass extinction (LOME) included two phases (I and II) of high species turnover that have been hypothetically linked to the Hirnantian glaciation and subsequent rapid warming, respectively. However, the timing and tempo of the LOME remain uncertain, which hinders our understanding of the feedback between the LOME and paleoclimatic change. Here, we present high-precision radioisotopic dates for the Ordovician-Silurian transition in South China that reveal the LOME began at 442.76 + 0.35/-0.22 million years ago, with the two phases lasting for 0.34 + 0.46/-0.34 and 0.06 + 0.31/-0.06 million years, respectively. The rapid switch from icehouse to greenhouse conditions, along with the higher mean rate of temperature change during LOME II, resulted in a much higher mean extinction rate during LOME II than I (71.6% versus 8.4% species loss per 100 thousand years, respectively), implying that the rate of climate change was a primary control on the tempo of the LOME.