The design of an efficient synthesis for large-scale production of atomically precise nanoclusters (NCs) is pivotal in realizing the size-dependent properties of the NCs. A simple and versatile method for producing atomically precise thiolated gold NCs (Au(25)(SR)(18) NCs) in large quantities (~200 mg) is demonstrated in this study. It uses a gaseous reducing agent, carbon monoxide (CO), to support a slow and size-controlled growth of Au(25)(SR)(18) NCs. Absorption measurements of the reaction solution, which underwent distinct color changes (colorless → yellow → orange → brown → red-brown), allowed the formation of thiolated Au(25) NCs to be reconstructed from several key intermediates. The unique reaction environment provided by gaseous CO presents a new synthetic route to fabricate atomically precise metal NCs in quantities large enough for application explorations.