Plasmonic photocatalysis offers a promising route to harvest solar energy and drive chemical reactions by the generated heat and energetic charge carriers towards a low-carbon economy. Understanding the mechanisms of plasmonic photocatalysis is vital for the rational design of efficient catalysts but remains a great challenge. Here we report the discovery of an overlooked photocatalytic mechanism based on the enhanced coverage of CO on an Au-Ni heterostructure under light. Detailed studies reveal that hot carrier transfer from Au to Ni via plasmonic excitation increases the electron density of Ni, thereby boosting the population of Ni sites capable of CO adsorption. This increase in active CO-binding sites, rather than local heating or altered adsorption properties of individual Ni sites, is found to be the major contributor to the light-promoted CH₄ production in the Au-Ni-catalyzed CO hydrogenation reaction. This mechanism is further validated by the observation of higher CH₄ selectivity in photocatalytic CO₂ hydrogenation. Our study deepens the mechanistic understanding of plasmonic photocatalysis and offers new ideas for the design of advanced catalytic structures.