A wideband (0.01–1000 s) magnetotelluric survey across the southern Sierra Nevada has identified zones of enhanced conductivity in the lower crust and upper mantle that underlie the resistive batholithic rocks at depths greater than 10–20 km. The eastern zone underlies the highest topography of the range and extends eastward. This eastern conductive zone extends to depths in excess of 100 km, based on model sensitivities, and lies well below the Moho depth of 32–38 km from seismological studies. Therefore the enhanced conductivity in the mantle cannot be attributed to conventional explanations for conductive continental crust and is instead likely due to partial melt. Such an interpretation is consistent with gravitational, seismological, and geologic evidence. Estimates of the partial melt fraction range from 2 to 5% at depths of 40–70 km, which are consistent with fractions of melt inclusions observed in xenoliths from the eastern Sierra Nevada. This partial melt accounts for approximately one third of the density decrease required in the mantle for support of the present high elevations. The other two thirds of the density decrease could be due to thermal expansion of the upper mantle or due to mineralogical changes. The western conductive zone also straddles the Moho and extends into the mantle beneath western foothills of the Sierra Nevada and the Great Valley sediments to the west. However, xenoliths from this region indicate high‐velocity crustal rocks to depths in excess of 60 km, and we therefore attribute the enhanced conductivity to graphitic metasediments and/or dehydration of metaserpentinite emplaced by downward return flow of the host rocks during intrusion of the Sierran plutons.