Studies attempting to gain new insights into the last stage of the subduction cycle are typically challenged by limited direct observations owing to a lack of recent post-subduction settings around the world. Central to unravelling how the subduction cycle ends is an understanding of crust and mantle processes that take place after subduction termination. Northern Borneo (Malaysia) represents a unique natural laboratory because it has been the site of two sequential subduction episodes of opposite polarity since the mid-Paleogene. The region exhibits several enigmatic post-subduction (after ~10 Ma) features, including: subsidence followed by rapid uplift, localised intraplate volcanism, possible orogen collapse, and a pluton that emerged to become the third highest peak in southeast Asia, Mt Kinabalu (4095 m). Arrival-time residuals from distant earthquake data recorded by the nBOSS seismic network have been used to investigate P- and S-wavespeed variations in the crust and underlying upper mantle beneath northern Borneo. Our 3-D tomographic images consistently show a high-velocity perturbation in western Sabah that we associate with an upper-mantle remnant of the Proto South-China Sea slab, thus providing important constraints for tectonic reconstructions of SE Asia. The tomographic models, combined with other seismological and geological information, reveal evidence for lithospheric removal in eastern Sabah via a drip instability. Our results suggest that lithospheric drips can be smaller than previously thought, yet their effects on the post-subduction evolution of continental lithosphere can be significant.