Shape- and crystal structure-dependent exciton dynamics of semiconducting nanoparticles were investigated using time- and polarization-resolved magneto-photoluminescence spectroscopy. Experiments were performed at low temperature (<2 K) in magnetic fields up to 17.5 T to investigate spin-dependent radiative relaxation in CdSe quantum dots and nanorods. In contrast to 0-D quantum dots, one-dimensional CdSe nanorods displayed a high degree of spin polarization even at relatively low magnetic field strengths (2 T). The observed spin polarization, evident in intensity-integrated and wavelength-resolved photoluminescence and time-correlated single-photon counting measurements, resulted from spin-polarized excitons that were formed upon highly efficient mixing of “dark” and “bright” fine-structure states. The relative population of these fine-structures was dependent upon the strength of the applied magnetic field. These results demonstrate the potential use of particle shape and crystal structure, two key nanostructure design parameters, to control photoinduced nanoscale dynamics. The findings may impact significantly technologies such as solar-to-electric energy conversion, spintronics, and chemical lasers, which all employ the nanocrystal platform.