ZnS nanoparticles have been synthesized on vertically aligned carbon nanotubes by gas-phase conversion of ZnO nanoparticles which have been tethered on vertically aligned carbon nanotubes using atomic layer deposition (ALD). The resulting ZnO@CNT nanocomposite has been converted to ZnS@CNT by reacting it with hydrogen sulfide using thioacetamide as a precursor. The composition of the resulting nanocomposite could be tuned from a mixed ternary ZnS/ZnO@CNT nanocomposite to a pure ZnS@CNT nanocomposite. At the same time, the amount of wurtzite and sphalerite phases varies in the ZnS@CNT nanocomposite. The resulting nanocomposites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), selected area electron diffraction (SAED), ultraviolet-visible diffuse reflectance spectroscopy (UV-VIS DRS) and photoluminescence spectroscopy (PL). Finally, the different nanocomposites were tested for their photocatalytic activity by the photocatalytic decomposition under visible light using methyl orange (MO). Herein a systematic study of the photocatalytic activity of different compositions of ZnS in the ZnS@CNT nanocomposite was performed for the first time.