Rational design of highly efficient photocatalysts based on molybdenum cocatalysts is prospective to achieve hydrogen evolution under visible light. Herein, Mn_0.5Cd_0.5S nanoparticles decorated with O-doped MoS₂ nanosheets (denoted as O-MoS₂/Mn_0.5Cd_0.5S) were successfully synthesized. The incorporation of O atoms, which could trigger defects and enlarge the interlayer distance of MoS₂, was achieved by a simple hydrothermal method. The performances of O-MoS₂/Mn_0.5Cd_0.5S composites for catalyzing hydrogen evolution are rapidly enhanced with increasing contents of O-MoS₂. The resultant 12 wt% O-MoS₂/Mn_0.5Cd_0.5S composite exhibits superior stability and a satisfactory hydrogen production rate up to 84.32 mmol g⁻¹ h⁻¹ and the apparent quantum efficiency at 420 nm is 46.92%, which is 3.18 times higher than that of Mn_0.5Cd_0.5S nanoparticles. To the best of our understanding, such an exceptional activity of 12 wt% O-MoS₂/Mn_0.5Cd_0.5S prepared in this work for hydrogen evolution is one of the highest among all reported Mn–Cd–S-based catalysts. The markedly strengthened activity is attributed to the improved light absorption and effective separation of photoinduced carriers between O-MoS₂ nanosheets and Mn_0.5Cd_0.5S nanoparticles. This work indicates that O-MoS₂ is a potential cocatalyst for the rational construction of efficient photocatalysts.