g-C3N4 layers with good mechanical properties, including their cohesion and adhesion to stainless-steel supports, were prepared by the quantitative electrophoretic co-deposition of g-C3N4 and WO3 nanocrystals. It was carried out in a mixture of organic solvents by applying a voltage of 750 V. The typical layer area density was 0.64 mg cm–2. The photocatalytic degradation of 4-chlorophenol under blue-light irradiation showed that the performance of stable composite layers containing 25–50 wt.% of WO3 was only slightly weaker than that of unstable pristine g-C3N4 ones. The high photocatalytic performance was due to g-C3N4, while WO3 contributed to a good mechanical resistance of layers in stirred water. Finally, the composite layers exhibited a very high 4-chlorophenol mineralization of 75% in 24 h, even higher than the corresponding suspensions. Owing to their stability in water and performance, the developed layers are suitable for applications in environmental technologies.