In this study, Ni-doped CdS photocatalysts immobilized on waste-derived activated carbon (AC) were successfully synthesized using spent coffee grounds as a sustainable carbon precursor. The composites were prepared via an in situ hydrothermal sulfuration method, and Rhodamine B (RhB) was employed as a probe molecule to investigate interfacial charge-transfer behavior under visible-light irradiation. Structural and morphological analyses confirmed the successful immobilization of CdS and Ni-doped CdS nanoparticles onto the porous carbon matrix, while no secondary Ni-containing phases were detected. Optical studies revealed that Ni incorporation induced a red shift in the absorption edge and narrowed the optical band gap from 2.23 eV for CdS/AC to 2.14 eV for Ni(5%)-CdS/AC. Photoluminescence measurements showed a moderately suppressed and slightly red-shifted emission for the Ni-doped sample, indicating reduced electron–hole recombination and improved charge separation. Photocatalytic experiments demonstrated that Ni(5%)-CdS/AC achieved an RhB degradation efficiency of approximately 85% within 60 min, significantly outperforming CdS/AC under identical conditions. Reactive species trapping experiments identified photogenerated holes (h⁺) and superoxide radicals (•O₂⁻) as the dominant active species in the degradation process (h⁺> •O₂⁻> •OH). The enhanced photocatalytic performance is attributed to the synergistic effects of Ni-induced electronic structure modulation and efficient interfacial charge transfer facilitated by the conductive activated carbon support. Beyond using waste-derived carbon merely as a support, this study provides mechanistic evidence—via optical analyses and scavenger tests—clarifying the dominant reactive species and the charge-separation role in Ni–CdS/AC under visible light. This work highlights an effective strategy for designing high-performance, waste-derived carbon-supported photocatalysts for visible-light-driven environmental remediation.