Clay-supported heterojunctions were assessed using ultrathin g-C3N4 as the light absorber, g-C3N4/CoAl2O4 heterojunction, and a ternary Laponite/CoAl2O4/UT g-C3N4 architecture. The UT g-C3N4 baseline improves photon capture and carrier mobility. Adding CoAl2O4 forms a direct heterojunction that drives interfacial charge separation and reduces recombination. By leveraging the dual-charged nature of Laponite, the sandwich-type Laponite/CoAl2O4/UT g- C3N4 catalyst promotes the formation of internal electrostatic microfields that facilitate directional charge migration across the UT g-C3N4/CoAl2O4 heterojunction, thereby suppressing electron–hole recombination and enhancing overall catalytic activity. The combined effects of heterojunction formation, and clay-enabled charge transport account for the observed stepwise enhancement from single to binary to ternary configurations. This concise design pathway illustrates how ultrathin g-C3N4, spinel coupling, and a clay scaffold can be integrated into a scalable sandwich-type photocatalyst for efficient solar hydrogen evolution. Under visible-light irradiation, g-C3N4-NS delivered a hydrogen evolution rate of 0.12 mmol g−1 h−1 and an STH efficiency of 0.25%, while UT g-C3N4 reached 0.18 mmol g−1 h−1 with an STH efficiency of 0.37%. The UT g-C3N4/CoAl2O4 heterojunction achieved 0.61 mmol g−1 h−1 with an STH efficiency of 1.23%, and incorporation of Laponite raised the rate to 1.50 mmol g−1 h−1 with an STH efficiency of 3.6%. These values correspond to 2.5-fold and 12-fold improvements over the binary and pristine systems, respectively. The optimized ternary catalyst sustained a linear hydrogen evolution profile for 24 h and reached a cumulative yield of 27 mmol g−1 without deactivation.