The East African Rift System provides a rare location in which to observe a wide scope of rifting states. Well-defined active narrow rifting in the Main Ethiopian Rift (MER) transitions to incipient extension and eventually pre-rifted lithosphere through the northwestern flank of the Ethiopian Plateau (EP). Although the MER is well studied, the off-axis region has received less attention. We develop Rayleigh wave phase velocity maps, Ps receiver functions, and H-κ; stack surfaces, and jointly invert these data using a trans-dimensional, hierarchical Bayesian inversion algorithm to create shear velocity profiles across the MER and EP. All shear velocities observed are slower than the PREM global average, a reflection of the elevated temperatures that persist from plume impingement. In the EP, we find a shallow mantle slow shear velocity lineament parallel to the MER axis, amidst otherwise faster shear velocities. The crust is shallow in the MER, but also in the northwestern EP flank, with thicker crust found throughout the plateau caused by crustal underplating and flood basalt emplacement. Shear velocities more reduced than the already low regional average, in concert with surficial volcanic features, geodetic observations, and slow P- and S-wave anomalies, support off-axis extension in the Ethiopian plateau, requiring reevaluation of the localization of continental breakup in the narrow MER.