A simple and practical method is presented for the fabrication of mirror-like reflective nickel sulfide (NiS) films via a hydrothermal reaction to serve as efficient transition metal chalcogenide catalysts. The NiS films on fluorine-doped fin oxide (FTO) glass substrates exhibit dual-phase crystalline structure with tunable alpha-NiS/Ni3S4 composition. The alpha-NiS/Ni3S4 ratio could be tuned by varying the concentration of the sulfur source, thioglycolic acid in the reaction mixture. The key role of thioglycolic acid is to control the morphology and crystal phase structure as well as determine the nickel to sulfur ratio. The NiS films, especially those fabricated with an adequate amount of the sulfur source, show multifunctional properties with significantly improved electrical conductivity, catalytic activity, and light reflectance due to the multiple layers of the stacked structure. Owing to the unique features of the fabricated films, solid polymer electrolyte dye-sensitized solar cells (DSSC) based on NiS counter electrodes exhibit photovoltaic energy conversion efficiency of 6.22%, which is much higher than that achieved with commercial Pt counter electrodes (5.42%). This superior performance indicates the immense potential of the NiS film as an efficient catalyst alternative for DSSCs.