Garnet-type Li5La3Nb2O12, among several other Li-stuffed garnet compounds, is known to be unstable under moisture and undergoes H+/Li+ ion-exchange at room temperature. While there have been reports on proton-exchange in water, the transport of the protons in the garnet structure is yet to be characterized by electrical methods. In this work, proton-exchange was performed on the pellet form of Li5La3Nb2O12 for 4, 7, and 10 days, while the pellet generally showed less proton-exchange due to less surface area exposed to water, longer treatment times did allow the reaction to proceed further. Neutron diffraction (ND) studies were utilized to locate the H-site in the garnet-type structure and to confirm which Li ion sites were exchanged. For H exchanged Li5La3Nb2O12, the neutron diffraction studies indicated that H exchanged preferentially for Li in the distorted octahedral sites of the Iad space group (no. 230). AC impedance measurements were done on as-prepared Li5La3Nb2O12 under 3% H2O + N2 and 3% D2O + N2, which suggested proton conduction under humid conditions at 23–300 °C. Li5La3Nb2O12 shows a proton conductivity of 10−5 S cm−1, of the same order of magnitude as Li ion conduction, at room temperature in humidity where protons seem to be incorporated from the solid and gas phase equilibrium reaction. Proton reversible Nafion electrode experiments indicate facile proton conduction in Li garnets under humidity.