Li-Mg alloy electrodes are promising as anodes for Li-ion batteries due to the high Li storage capacity and the relatively lower volume change during the lithiation or delithiation process. They also present a unique opportunity to visualize and quantify or image the Li distribution through the thickness at various states of delithiation. In this work, spatial distributions of Li in electrochemically delithiated Li-Mg electrodes have been determined quantitatively using neutron tomography. Specifically, the Li concentration profiles along thickness direction are determined from the attenuated neutron intensity. A rigorous analytical model to quantify the diffusion-controlled delithiation, accompanied with phase transition and boundary movement, has also been developed. The analytical scheme successfully predicted the Li concentration profiles which agreed well with the neutron imaging data. It is demonstrated that Li is depleted by diffusion through the solid solution Li-Mg phases and this proceeds with β→α phase transition and the associated phase boundary movement through the electrode. This is also accompanied by the thinning of the electrode due to the change in molar volume. Following the approach developed here, one can develop a rigorous and quantitative understanding of Li depletion in electrodes of electrochemical cells, in addition to the present Li-Mg alloy electrodes.