The functionality of liquid–liquid interfaces formed between two immiscible electrolyte solutions (ITIES) can be markedly enhanced by modification with supramolecular assemblies or solid nanomaterials. The focus of this Review is recent progress involving ITIES modified with floating assemblies of gold nanoparticles or “nanofilms”. Experimental methods to controllably modify liquid–liquid interfaces with gold nanofilms are detailed. Also, we outline an array of techniques to characterize these gold nanofilms in terms of their physiochemical properties (such as reflectivity, conductivity, catalytic activity, or plasmonic properties) and physical interfacial properties (for example, interparticle spacing and immersion depth at the interface). The ability of floating gold nanofilms to impact a diverse range of fields is demonstrated: in particular, redox electrocatalysis, surface-enhanced Raman spectroscopy (SERS) or surface plasmon resonance (SPR) based sensors, and electrovariable optical devices. Finally, perspectives on applications beyond the state-of-the-art are provided.