We investigated the dilatational rheology and dynamic interfacial tension of soy β-conglycinin (7S) and soyasaponin (Ssa) mixtures at the oil-water interface, followed by the formation and short/long-term stability of the subsequent emulsions to evaluate the mechanism(s) of stabilizing emulsions using a fixed concentration of 7S (0.5%) in all the samples. The presence of a low concentration of Ssa (0.05%) significantly affected the interfacial dilatational and emulsification properties although 7S dominated the interface. Higher concentrations of Ssa (0.1%–0.75%) showed synergistic effects on the changes in interfacial tension due to the interactions between 7S and Ssa. These effects were primarily attributed to the complexation of 7S with Ssa, thereby loosening the rigid conformation of the protein and unfolding 7S at the interface. Owing to the elasticity of the mixed adsorption layers, the interfacial 7S–7S and 7S-Ssa interactions were enhanced. The improved resilience to external deformations and interfacial behaviors of these mixed layers induced long-term stability (42 days) in these emulsions. These results provide a potential application for natural food additives in designing emulsions by modulating protein-saponin mixtures to induce long-term stability.