A simple ultrasonic radiation method was employed for the preparation of zinc and cadmium sulfide solid solution (Zn_xCd_1−xS; x = 0–0.25 wt.%) with the aim to investigate its efficiency for H₂ production via a visible light-driven water-splitting reaction. The catalyst characterization by X-ray diffraction confirmed the formation of solid solution (Zn_xCd_1−xS) between CdS and ZnS phases. All catalysts exhibited hierarchical morphology (from SEM and TEM) formed by aggregated nanoparticles of Zn_xCd_1−xS solid solution with crystals showing mainly (111) planes of cubic CdS phase. The crystal size linearly decreased with an increase in Zn incorporation in the crystal lattice (from 4.37 nm to 3.72 nm). The Zn_xCd_1−xS photocatalysts showed a gradual increase in the H₂ evolution, with an increase in the Zn concentration up to 0.2 wt.% making the most effective Zn_0.2Cd_0.8S catalyst toward H₂ production. From the catalyst activity–structure correlation, it has been concluded that the twin-like CdS structure, the (111) plane and specific morphology are the main factors influencing the catalyst effectivity toward H₂ production. All those factors compensated for the negative effect of an increase in band gap energy (E_bg) after ZnS incorporation into solid solution (from 2.21 eV to 2.34 eV). The effect of the catalyst morphology is discussed by comparing H₂ evolution over unsupported and supported Zn_0.2Cd_0.8S solid solutions.