Iron‐doped tin oxide (Sn0.9Fe0.1O2), and specifically carbon‐coated Sn0.9Fe0.1O2 (Sn0.9Fe0.1O2‐C) provides high reversible capacity and a reasonably low de‐/lithiation potential owing to the combined conversion and alloying mechanism. The initial (quasi‐)amorphization during the first lithiation, however, renders an in‐depth understanding of the reaction mechanism challenging. Herein, a comprehensive investigation via a set of highly complementary characterization techniques is reported, including operando X‐ray diffraction, ex situ 119Sn and 57Fe Mössbauer spectroscopy, ex situ 7Li NMR spectroscopy, operando isothermal microcalorimetry (IMC) of Li‖Sn0.9Fe0.1O2‐C coin cells, and electrochemical microcalorimetry of single Sn0.9Fe0.1O2‐C electrodes. The combination of these advanced techniques allows for detailed insights into the lithiation and delithiation mechanism and the potential determining processes, despite the (quasi‐) amorphous nature of the active material after the initial lithiation.