This manuscript presents a facile route for the preparation of ZnO–C nanocomposites using zinc citrate dihydrate as a precursor through one-step calcination under a nitrogen atmosphere. The products were characterized by X-ray diffraction (XRD), energy-dispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), Raman spectroscopy and ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS). The results showed ZnO nanoparticles, 5–10 nm in size, were distributed on the carbonaceous layers to form the ZnO–C nanocomposites. The as-prepared ZnO–C nanocomposites exhibited enhanced photocatalytic activity compared to pure ZnO with respect to methylene blue (MB) degradation under UV irradiation, which was attributed to the improved separation of photogenerated electrons and holes in the presence of carbonaceous layers. In particular, the rate of degradation of methylene blue (MB) with ZnO–C(550N) as a photocatalyst was 4.15 times faster than that of using bare ZnO nanoparticles. Furthermore, the ZnO–C nanocomposites could be easily recycled without an obvious decrease in photocatalytic activity. The resulting ZnO–C nanocomposites have potential applications in photocatalysis and environmental protection.