We investigated the photocatalytic behavior of gold nanoparticles supported on CeO₂–TiO₂ nanostructured matrixes in the CO preferential oxidation in H2-rich stream (photo-CO-PROX), by modifying the electronic band structure of ceria through addition of titania and making it more suitable for interacting with free electrons excited in gold nanoparticles through surface plasmon resonance. CeO₂ samples with different TiO₂ concentrations (0–20 wt %) were prepared through a slow coprecipitation method in alkaline conditions. The synthetic route is surfactant-free and environmentally friendly. Au nanoparticles (<1.0 wt % loading) were deposited on the surface of the CeO₂–TiO₂ oxides by deposition–precipitation. A benchmarking sample was also considered, prepared by standard fast coprecipitation, to assess how a peculiar morphology can affect the photocatalytic behavior. The samples appeared organized in a hierarchical needle-like structure, with different morphologies depending on the Ti content and preparation method, with homogeneously distributed Au nanoparticles decorating the Ce–Ti mixed oxides. The morphology influences the preferential photooxidation of CO to CO₂ in excess of H₂ under simulated solar light irradiation at room temperature and atmospheric pressure. The Au/CeO₂–TiO₂ systems exhibit much higher activity compared to a benchmark sample with a non-organized structure. The most efficient sample exhibited CO conversions of 52.9 and 80.2%, and CO₂ selectivities equal to 95.3 and 59.4%, in the dark and under simulated sunlight, respectively. A clear morphology–functionality correlation was found in our systematic analysis, with CO conversion maximized for a TiO₂ content equal to 15 wt %. The outcomes of this study are significant advancements toward the development of an effective strategy for exploitation of hydrogen as a viable clean fuel in stationary, automotive, and portable power generators.