Herein, we describe a simple solid-state synthesis of zinc sulfide (ZnS) and its amorphous carbon–modified nanocomposite (ZnS/C) and examine their performance as high-capacity anode materials for rechargeable Li-ion batteries, probing lithiation/delithiation mechanisms by ex situ X-ray diffraction and extended X-ray absorption fine structure analyses. The ZnS converted into LiZn and Li2S during lithiation, being subsequently incompletely recombined into ZnS during delithiation and thus resulting in traces of Zn and S. Based on the above investigation of reaction mechanism, the ZnS/C electrode was electrochemically tested within a potential range of 0–2.0 V vs. Li+/Li and exhibited a high reversible capacity of 681 mAh g−1, excellent cycling stability (>150 cycles), and high rate capability.