We take two typical Northeast Asia bimodal volcanoes as examples to explain the general features of Cenozoic bimodal ocean island basalt (OIB)-type volcanism in Northeast Asia. We present mineralogical, petrological, geochemical, isotopic, and full-waveform seismic tomographic evidence for the existence of two-layer magma chambers of Late Cenozoic volcanic activity beneath Ulleung Island and Mt. Changbai (Paektu). Ulleung Island volcanic rocks, which are composed of alkaline basalt, phonotephrite, trachyte, and phonolite, belong to the alkaline magma series and display enrichment of light rare earth elements (LREEs) and large ion lithophile elements (LILEs), slight depletion of heavy rare earth elements (HREEs), enriched ⁸⁷Sr/⁸⁶Sr (0.70475–0.70507) and ¹⁴³Nd/¹⁴⁴Nd (0.51250–0.51255) isotopic values, and enriched ²⁰⁷Pb/²⁰⁴Pb (15.544–15.626) and ²⁰⁸Pb/²⁰⁴Pb (38.750–38.954) values, similar to the geochemistry of OIB. Ulleung Island felsic volcanic rocks are characterized by significant negative Ba, Sr, P, Eu, and Ti anomalies (δEu = 0.14–0.35) and positive Pb anomalies, slightly higher ⁸⁷Sr/⁸⁶Sr isotopic ratios relative to those of mafic volcanic rocks, although the mafic and felsic samples have similar Sr, Nd, and Pb isotope compositions without significant differences. Ulleung Island and Mt. Changbai volcanic activities are likely related to the involvement of subduction-related compositions, but parts of the Mt. Changbai samples have been contaminated by crustal components to a certain extent. Mafic volcanic rocks of Ulleung Island are segregated from a deeper mantle source within a pressure range of 10.1–21.2 kbar compared with felsic volcanic rocks, which exhibit fractional crystallization of clinopyxene, spinel, plagioclase, and olivine. To explain this, a two-layer magma chamber beneath Ulleung Island with depths of 30–40 and 60–80 km is proposed, which is supported by mineral crystallization pressure and a three-dimensional full-waveform seismic tomography model. We also suggest that similar magma eruption processes and a two-layer magma chamber with depths of ~10 and 40–60 km also exist beneath Mt. Changbai. Taking the typical Cenozoic bimodal samples from Ulleung Island and Mt. Changbai as examples, we argue that two-layer magma chambers exist beneath Cenozoic bimodal OIB-type volcanic activities in Northeast Asia.