A complete mechanistic study on the nucleation of polymeric nanoparticles covering the generation of the clusters and the forthcoming aggregation to the nuclei is performed by in situ 1H nuclear magnetic resonance (NMR) experiments using polyaniline as an example. An aniline tetramer and a monomer-stabilized nitrenium are proved to be the basic cluster and predominant propagating intermediate in the nucleation. It is observed that the nuclei are generated via a sequential mechanism involving a translocation of the protonated tetramers to the aqueous bulk, dissociation of sodium dodecyl sulfate (SDS) micelles, and deprotonation to induce the fusion of the dissociated micelles and intermolecular packing of the oligomers. Despite its importance, direct observation of the nucleation is challenging. This work emphasizes the importance of utilizing the solvent in solvation shell as the sensitive probe to explore the most transient process in nucleation, demonstrating its efficiency in achieving information such as the stepwise procedures, the nuclei sizes, the growth kinetics, and so forth. The approach reported herein may prove of great value in establishing the missing link of the atomic origin of nanoparticles, a key topic toward the preparation of functional nanostructures with well-controlled architectures, and can be readily extended to the study of other organic or inorganic systems.