Ancient cratons are typically characterized by thick lithospheric-mantle roots that provide them with buoyancy and rigidity. Once formed, cratons, including their roots, move around Earth's surface as parts of rigid plates that are normally unaffected by later magmatism or tectonic activity. Considerable geophysical and geochemical evidence, however, suggests that the ancient lithospheric mantle beneath the Chinese portion of the eastern Sino-Korean craton was replaced by thinner, hotter, juvenile mantle during the Jurassic to Early Cretaceous. The extent and cause of the changes to this lithospheric mantle have been debated. Keys to deciphering the history of the changes include knowledge of both the lateral extent of the lithospheric modifications and the relative timing of change across the craton. Discerning the age and structure of lithospheric mantle underlying North Korea, the easternmost part of the Sino-Korean craton, is thus particularly important. Here we report mineral compositions and Re-Os isotopic data for peridotite xenoliths from North Korean Triassic kimberlite and Tertiary basalts. The data suggest that, from the Triassic to the present, North Korea has been underlain by young, hot and fertile lithospheric mantle, unlike typical cratonic lithospheric roots, but similar to the juvenile lithospheric mantle underlying Chinese portions of the craton. Given the generally ancient nature of the crust composing North Korea, our findings suggest that modification of underlying cratonic roots extended to the eastern edge of the craton. The Triassic eruption age of the kimberlite suggests that the lithospheric changes may have occurred earlier than in China, indicating that lithospheric removal evolved from east to west. These results are most consistent with the conclusion that lithospheric loss was initially triggered by extension that followed the collision between the Sino-Korean and Yangtze cratons.