The advancement of direct solar-to-fuel conversion technologies requires the development of efficient catalysts as well as efficient materials and novel approaches for light harvesting and charge separation. We report a novel system for unprecedentedly efficient (with near-unity quantum yield) light-driven reduction of methylviologen (MV(2+)), a common redox mediator, using colloidal quasi-type II CdSe/CdS dot-in-rod nanorods as a light absorber and charge separator and mercaptopropionic acid as a sacrificial electron donor. In the presence of Pt nanoparticles, this system can efficiently convert sunlight into H(2), providing a versatile redox mediator-based approach for solar-to-fuel conversion. Compared to related CdSe seed and CdSe/CdS core/shell quantum dots and CdS nanorods, the quantum yields are significantly higher in the CdSe/CdS dot-in-rod structures. Comparison of charge separation, recombination and hole filling rates in these complexes showed that the dot-in-rod structure enables ultrafast electron transfer to methylviologen, fast hole removal by sacrificial electron donor and slow charge recombination, leading to the high quantum yield for MV(2+) photoreduction. Our finding demonstrates that by controlling the composition, size and shape of quantum-confined nanoheterostructures, the electron and hole wave functions can be tailored to produce efficient light harvesting and charge separation materials.