The pathways and dynamics of converting spin-allowed (S = 0) singlet excitons to spin-forbidden (S = 1) triplets have significant implications in determining performance metrics of conjugated polymers in optoelectronic devices. We study the effect of structural ordering factors on triplet formation in self-assembled aggregate π-stacked chains of poly­(3-hexylthiophene) (P3HT) using single-molecule time-resolved intensity modulation and electric-field-dependent photoluminescence (PL) spectroscopy. Triplet generation is only efficient in P3HT aggregates of high purity, and formation yields are found to increase with nanofiber size. We propose that the high intrachain order in purified aggregates that extends exciton coherence lengths, leading to J-aggregate spectral signatures, is also important for populating interchain charge transfer (CT) states that, at longer times, recombine preferentially to triplets according to spin statistics. Electric-field-dependent PL decays of isolated P3HT aggregates show large modulation of a long-lived emitting state attributed to the delocalized intrachain exciton with substantial CT state admixture. Our results demonstrate the importance of dark CT states in mediating exciton relaxation and spin conversion processes that are usually obscured in conventional thin films by heterogeneity. We further demonstrate the utility of subtle structural factors for selecting photophysical outcomes by careful control of processing conditions.