Self-assembly of conjugated organic semiconductors into ordered, larger scale entities is a critical process to achieve efficient charge transport at the nano-through macro-scales, and various methodologies aimed at enhancing molecular ordering have been introduced. However, mechanistic understanding is limited. Here, a mechanistic elucidation of poly(3-hexylthiophene) (P3HT) molecular self-assembly is proposed based on experimental demonstration of controlled, solution-based P3HT self-assembly into rod-like polycrystalline nanostructures. The synergistic combination of nonsolvent addition and ultrasonication facilitates rod-like P3HT nanostructure formation in solution. Importantly, through sequential application of both treatments, nanostructure length can be easily modulated, and the assembly process is shown to follow a simple 2-step crystallization model, which depends upon nucleation followed by growth. Through arrays of experimental results, the validity of 2-step crystallization is confirmed and is proposed as a comprehensive platform to understand self-assembly processes of conjugated polymers into larger, ordered mesoscale entities.