The detailed understanding of kinetic and phase dynamics taking place in lithium-ion batteries (LIBs) is crucial for optimizing their properties. It was previously reported that Fe_1-xS/C nanocomposites display a superior performance as anode materials in LIBs. However, the underlying lithium storage mechanism was not entirely understood during the 1st cycle. In this work, in operando synchrotron techniques are used to track lithium storage mechanisms during the 1st (de)-lithiation process in the Fe_1-xS/C nanocomposite. The combination of in operando techniques enables the uncovering of the phase fraction alternations and crystal structural variations on different length-scales. Additionally, the investigation of kinetic processes, morphological changes, and internal resistance dynamics is discussed. These results reveal that the phase transition of Fe_1-xS → Li₂Fe_1-xS₂ → Fe⁰ + Li₂S occurs during the 1st lithiation process. The redox reaction of Fe²⁺ + 2e^- ⇌ Fe⁰ and the Fe K-edge X-ray absorption spectroscopy (XAS) transformation process are confirmed by in operando XAS. During the 1st de-lithiation process, Fe⁰ and Li₂S convert to Li_2-yFe_1-xS₂ and Li⁺ is extracted from Li₂S to form Li_2-yS. The phase transition from Li₂S to Li_2-yS is not detected in previous reports. After the 1st de-lithiation process, amorphous lithiated iron sulfide nanoparticles are embedded within the remaining Li₂S matrix.