Molybdenum rich molybdenum carbide (Mo-Mo₂C) has recently been confirmed as a highly effective non-noble-metal cocatalyst to boost the photocatalytic activity of g-C₃N₄ for water splitting. However, the activity of Mo-Mo₂C is still much lower than that of Pt noble metal based cocatalysts for photocatalytic hydrogen production (PHE). This work reports a highly active cobalt (Co) doped Mo-Mo₂C cocatalyst (Co(Mo-Mo₂C)) for PHE. The hydrogen production rate of 2.0 wt.% optimal Co(Mo-Mo₂C) loaded g-C₃N₄ (2.0 wt.% Co(Mo-Mo₂C)/g-C₃N₄) photocatalyst can be as high as 11,291 μmol h⁻¹ g⁻¹, at 6.7% apparent quantum efficiency (AQE) at 420 nm, which is 1.4 times greater than that of 2.0 wt.% Pt/g-C₃N₄ photocatalyst. Catalyst characterizations have pointed out that Co doped Mo-Mo₂C cocatalyst is able to efficiently suppress photo-generated charge recombination, thus improving the interfacial charge transfer rate. This research provides novel ideas and technical approaches for designing a noble-metal-free, efficient and environmentally friendly cocatalyst for photocatalytic H₂ evolution via water reduction.