In this work, a ternary heterojunction photocatalyst composed of mesoporous graphitic carbon nitride, black phosphorus, and molybdenum disulfide (m-CN/BP/MoS2) was firstly fabricated for the visible-light-driven hydrogen evolution reaction (HER) via water splitting. The photocatalytic activity of m-CN/BP/MoS2 photocatalyst was further improved by incorporating Ni and Co co-catalysts, denoted as m-CN/BP/MoS2-Y (Y: Ni, Co). The final quaternary nanocomposites were experimentally demonstrated in the form of mixed heterojunctions (type-I, type-II, and Schottky junctions) to manipulate the photogenerated carriers with the aim of reducing their recombination rate and increasing the light-harvesting ability of the pristine components. To find the best combination of different semiconductors in the heterojunction to ensure the highest photocatalytic HER activity, the loading ratio of each component was optimized one-by-one by considering the rate of HER under the same conditions. The photocatalytic HER activity of m-CN/BP/MoS2-Ni (5 wt%) and m-CN/BP/MoS2-Co (0.5 wt%) photocatalysts reached up to 21.668 mmol g−1 and 29.179 mmol g−1 for 8 h in the presence of triethanolamine electron donor under visible light irradiation, which were about 11 and 15 times higher than that of m-CN/BP binary heterojunctions, 4 and 5 times higher than that of m-CN/BP/MoS2 ternary heterojunctions, respectively. Ni or Co co-catalysts enhanced charge separation and thus increasing the rate of H2 evolution owing to their suitable D-band levels. This present study supplies a strategy to design efficient photocatalysts for solar fuel production without using non-noble metal cocatalysts.