Continental cratons are characterized by thick lithospheric roots that remain intact for billions of years. However, some cratonic roots appear to have been removed, with the reasons for such removal being debated. In this study, we obtain a high-resolution full-waveform seismic tomographic model for North America which newly illuminates an ongoing craton-thinning process. Extensive dripping lithosphere is imaged from the base of the craton beneath the central United States to the mantle transition zone. From geodynamical modeling, we find such dripping can be mobilized by the sinking of the deep Farallon slab, which is presently not connected to the surface and located in the lower mantle. Dripping lithosphere could be further facilitated by past lithospheric weakening processes due to volatiles released from the slab. These findings show that cratonic lithosphere can be altered by external forces, and that subducting slabs play a key role in craton mobilization and thinning even when slabs are at great depths in the mantle.