Accurate knowledge of lithium content within the solid electrolyte interphase (SEI) layer and anode would significantly enhance the current understanding of the lithium ion battery (LIB) degradation mechanisms, enabling knowledge-based improvements in the technology. For the first time, we have demonstrated the capabilities of highly selective Lithium Nuclear Reaction Analysis (Li-NRA) as a non-destructive depth profiling technique for quantifying Li within the SEI and anode without accurate knowledge of the composition, which is unavailable with other depth profiling techniques. The Li-NRA technique detects the gamma radiation resulting from a nuclear reaction at characteristic resonance energy between an incident high-energy proton and Li. The intensity of γ-ray is directly proportional to the Li content, and the energy of the incident proton is increased stepwise to depth profile the sample. We performed Li-NRA on the carbonaceous negative electrodes of commercial LIB coin cells at varying states of charge (SOC) and states of health (SOH) conditions. We used three simple models for the composition of SEI and anode material to show concurrence between theoretical and experimental value for Li content at varying SOC conditions, estimated the average SEI layer thickness, and correlated the residual Li content within the SOH samples with electrochemical data.