A nonagon ZnO-ZnF2O4 nanocomposite was synthesized using high-energy ball milling followed by post-annealing with an initial mixture of high purity magnetite (Fe3O4) and zincite (ZnO) powders. The effects of the process parameters on the structural, optical, photocatalytic, and photoelectrocatalytic activities were investigated. X-ray diffraction analysis was used to confirm the presence cubic spinel ZnFe2O4 and hexagonal ZnO crystal phases. The lowest optical bandgap observed was 1.83 eV, which decreased due to post-annealing. The photocatalytic activity of the 800 degrees C post-annealed nanocomposite exhibited the highest degradation percentage of 96% over 180 min under visible light, with a degradation rate constant of 1.5 min(-1). Furthermore, the optimized sample showed no substantial decrease in photocatalytic activity after five cycles of the repeated catalysis experiment. In electrochemical studies, when compared to the other post-annealed samples, the 800 degrees C post-annealed nanocomposite showed the highest water splitting activity with the lowest charge-transfer resistance of 75 Q, photocurrent density of 4.3 x 10(-5) Acm(-2), exchange current density of 4.19 log JA(-1) cm(-2), and Tafel slopes of 74.1 mVdec(-1). In dual activities, the performance of the 800 degrees C post-annealed nanocomposite was noticed to be enhanced. This nanocomposite has potential industrial applications as a cost-effective catalyst.