Carbon dots (CDs) were successfully derived from water hyacinth leaves and the binary composite was achieved by incorporating CDs with g-C3N4 through hydrothermal treatment. The average particle size of CDs was found to be 3.1 nm and a blue-green fluorescence was emitted under the UV light irradiation. Both of the composites loaded with 20 wt.% (20CDs/g-C3N4) and 40 wt.% (40CDs/g-C3N4) of CDs achieved the highest degradation efficiency of 2,4-dichlorophenol (2,4-DCP) with 1.7 times higher than that of pure g-C3N4. This work successfully improved the properties of g-C3N4 by elongating the lifetime of photogenerated electrons and widening the visible light response. Both of 20CDs/g-C3N4 and 40CDs/g-C3N4 recorded the highest photocatalytic performance in degrading 2,4-DCP with degradation rate constant of 0.0194, and 0.0186 min−1, respectively. This is contributed by the prolonged charge carrier lifetime in 20CDs/g-C3N4; good visible light absorption and high specific surface area in 40CDs/g-C3N4. For the scavenger test, hole (h+) and superoxide radical (·O2−) were acknowledged as the key active species in photocatalysis.