Synergistically improved mechanical and dielectric properties of poly(lactic acid)/ethylene–methyl acrylate–glycidyl methacrylate random terpolymer (PLA/EMA-GMA) based nanocomposites containing ionic liquids (ILs) and multi-walled carbon nanotubes (MWCNTs) can be related to the morphological evolution and interface properties of the composites. ILs can catalyze the reaction between PLA and EMA-GMA and MWCNTs can induce morphology transformation from sea-island structure to quasi-cocontinuous structure. The rheological tests show that the G′ of samples significantly increased and G″ decreased with increasing the MWCNTs content. This indicates that a crosslink structure at PLA/EMA-GMA interface and a solid network structure of EMA-GMA/MWCNTs with quasi-cocontinuous morphology were formed by incorporation of ILs. Morphology transformation and interfacial enhanced compatibilization significantly improved the mechanical and dielectric properties of composites owing to the formation of MWCNTs network in EMA-GMA phase and interfacial crosslinked structure.