ZnxCo1−xS (0≤x≤1) nanoparticles were prepared by the hydrothermal method. The nanoparticles were characterized using XRD, EDX, FE-SEM, UV-Vis/DRS, VSM, and BET surface area analysis. A compositional phase evolution was observed, transitioning from pure cubic CoS (x=0) through a mixed CoS/ZnS phase (x=0.2), to pure cubic ZnS (x≥0.4). The morphology evolved from spherical nanoparticles to stacked layered structures with changing composition. ZnxCo1−xS shows soft ferromagnetic behavior, and the composition x=0.6 exhibited a maximum coercivity of 44 G and a saturation magnetization of 1.69 emu/g. A high surface area of 133.2 m2 /g was detected for pure CoS and systematically decreased with Zn content. The optical band gap has a minimum value of 3.31 eV for x=1 and a maximum value of 3.65 eV for x=0.5. The composition x=0.4 demonstrated exceptional photocatalytic activity, ⁓99.47%, towards Congo red within 75 min, while x=0.5 showed maximum adsorption capacity (34.36 mg/g) correlated with the minimal crystallite size and high surface area of ZnS phase. ZnxCo1−xS exhibits a tunable platform where composition directly controls structural, magnetic, optical, and catalytic properties, with x=0.4–0.6 emerging as the optimal multifunctional composition for environmental remediation applications.