Abstract Magnetotelluric (MT) surveys reveal high electrical conductivity anomalies along major trans‐lithospheric shear zones. Given that trans‐lithospheric shear zones are proposed conduits for deep‐Earth volatiles, we employ molecular dynamics simulations to quantify H+ diffusivity and electrical conductivity at olivine grain boundaries. Our simulations demonstrate that H+ diffusivity at grain boundaries exceeds lattice diffusivity by 4–5 orders of magnitude, establishing hydrogen as the dominant charge carrier. This enhanced grain boundary transport increases bulk olivine conductivity by approximately two orders of magnitude relative to anhydrous conditions. Critically, our calculated conductivities for realistic mylonitic microstructures (grain sizes 10–200 μm) quantitatively reproduce the high conductivity anomalies (0.01–1 S/m) observed in MT surveys of major shear zones worldwide. This consistency suggests that hydrogen‐bearing grain boundaries (H‐GB) could control shear zone conductivity. Our findings confirm that trans‐lithospheric shear zones constitute efficient pathways for mantle volatile migration, with implications for deep volatile cycling, metasomatism, and intraplate volcanism localization.