The effect of metal doping (i.e., with Cr, Fe, Ni, Cu, Zn, Ag and Sn) on the crystal structure of hydrothermally synthesized Mn1−xCdxS (where x≈0.1) is studied with the aim of enhancing photocatalytic activity. In contrast to the low-crystalline, undoped solid solution Mn1−xCdxS, Ni doping yields a well-crystallized wurtzite-type Mn-Cd-S solid solution, which precipitates as planar hexagonal facets of several hundred nanometers in size, together with much larger grains of α-MnS (>10 μm). By removing inactive α-MnS through sedimentation, a single phase with composition of Ni0.01Mn0.56Cd0.43S is obtained successfully. The Ni doping achieved a threefold higher photocatalytic activity for H2 evolution from a 0.1 M Na2S/0.5 M Na2SO3 solution under visible-light irradiation (λ≥420 nm). The apparent quantum yield of 1 wt % Pt-loaded Ni0.01Mn0.56Cd0.43S measured at λ=420 nm reached 25 %. The enhanced photocatalytic activity is most likely the result of a decreased concentration of defects, responsible for electron–hole recombination, in the active solid-solution phase and a slightly higher bandgap energy (2.4 eV).