A new organic-solvent-free heterologous redox mediator comprising ionic liquid [1-methyl-3-propylimidazolium iodide (MPII)], metal complex [tris(2-(1H-pyrazol-1-yl)-4-tert-butylpyridine)cobalt(III) tri[bis(trifluoromethane)sulfonimide] (FK209)], and additives, was applied to dye-sensitized solar cells (DSSCs). When MPII or FK209 as a redox mediator was individually used in combination with additives, the DSSCs recorded a low power conversion efficiency (PCE) of nearly 0%. In contrast, a DSSC with a mixture of MPII and FK209 exhibited a PCE of 9.63%, comparable to that of a device with a commercial organic-solvent-based I−/I3− electrolyte (9.67%). It was verified that a chemical reaction between MPII and FK209 led to the formation of I3−; therefore, three redox pairs, I−/I3−, I−/Co3+, and Co2+/Co3+, participated in hole transportation. Moreover, DSSCs with the heterologous redox mediator exhibited more efficient hole collection/dye regeneration and higher long-term stability than commercial organic-solvent-based cells.