An atom-by-atom ternary Cu–Mo–P nanocomposite was electrochemically co-deposited onto screen-printed carbon electrodes (SPCEs) to form a highly active sensing interface for the determination of metformin (MF). The one-step anodic fabrication yielded a porous, uniformly distributed network of mixed-valence Cu+/Cu2+ and Mo6+ centres embedded within a phosphate-stabilized matrix. Comprehensive microstructural and spectroscopic analyses revealed enhanced surface roughness, expanded electroactive area and favourable charge-transfer characteristics. The CuMoP/SPCE exhibited markedly amplified redox activity and proton-coupled electron-transfer behaviour toward MF, enabling sensitive detection with a linear range of 0.99–13.82 µM and a low detection limit of 0.85 µM. Excellent selectivity, stability and reproducibility were achieved, with recoveries of 94.32%–107.23% in pharmaceutical samples and negligible interference from common ions or saccharides. These results demonstrate the synergistic catalytic contributions of Cu, Mo and P in enabling efficient electron mediation, positioning the CuMoP/SPCE robust, low-cost platform for rapid pharmaceutical quality control and therapeutic monitoring.