Liquid–liquid interfaces between immiscible electrolytes constitute a defect-free, continuously renewed boundary that is well suited to probing interfacial electron transfer and catalysis. In this work, Cu and Pt nanoparticles are formed in situ on tungsten carbide (WC), a carbide support with Pt-like d-band electronic features and catalytic behavior, via interfacial electrodeposition, with decamethylferrocene supplying electrons from the organic phase. The interfacial fluidity promotes interfacial assembly of the deposits while limiting nanoparticle coalescence. Hydrogen evolution reaction (HER) activity is evaluated electrochemically using a four-electrode liquid–liquid configuration and chemically via a biphasic reaction, while kinetics are quantified by UV-Vis tracking. XPS, TEM, and SEM verify nanoparticle formation and the resulting WC/metal nanocomposite architectures. Compared with the uncatalyzed two-phase reaction, WC, WC/Cu, and WC/Pt increase HER rates by approximately 35-, 100-, and 75-fold, respectively. The superior WC/Cu performance is consistent with strong WC-Cu electronic coupling and favorable interfacial charge redistribution.