Ionic liquid (IL) based composite materials have shown great promise as antimicrobial agents, owing to their inherent germicidal properties, as well as their ability to stabilize metal nanoparticles (NPs), which may serve as a secondary antimicrobial reservoir. Here, we show that tetraalkylphosphonium ILs (TAPILs) on their own can annihilate pathogens by interfering with their cell membranes; however, the nature of the alkyl substituents on the central P atom and the nature of the anion play decisive roles in determining their antimicrobial activities. Concomitantly, TAPILs can stabilize copper nanoparticles (Cu NPs) generated directly within the IL matrices without the addition of any secondary stabilizers. The composites thus generated were thoroughly characterized and shown to be far more lethal to E. coli than just the TAPILs alone. The antibacterial effect demonstrated by the composite, composite-2, created from P[6,6,6,8]Cl (TAPIL-2) was orders of magnitude more lethal to microbes in comparison with P[6,6,6,8]Cl or copper nanoparticles alone. Neither the parent TAPIL-2 nor composite-2 were compromised by ambient storage conditions over a period of months with regards to their bactericidal effects. Composite-2 also proved to be effective against a panel of selected microbes. SEM studies were conducted to image E. coli after exposure to the TAPILs or composite-2; with the latter, only bacterial debris were noticed post-exposure, indicating total bacterial annihilation. The kinetic killing assay and regression analyses for time-dependent bactericidal activity of composite-2 against E. coli and S. aureus demonstrated increase in log reduction values over time, indicating the effectiveness of composite-2 in reducing the viable cell counts for both bacterial strains. Finally, Cu K-edge XANES was used to investigate the fate of Cu NPs within the composites, revealing oxidative disintegration of the Cu NPs within the TAPIL matrices over time. Copper ions and/or small copper clusters were released which interfere with the integrity and the permeability of E. coli cell membranes, inducing cell death. This was confirmed by SEM of bacterial preparations before and after exposure to both the TAPILs themselves as well as to the composites. Exposing E. coli to composite-2 causes complete cellular destruction, leaving behind cellular debris as the only visible organic matter. Thus, these TAPIL-based composites containing ‘ion reservoir’ metal NPs are potent antimicrobial materials, deserving additional research.