The formation of crystalline materials has been studied for more than a century. Recent discoveries about the self-assembly of many inorganic materials, involving aggregation of nanoparticle (NP) precursors or pre-nucleation clusters, challenge the simple assumptions of classical crystallization theory. The situation for organic materials is even more of a terra incognita due to their high complexity. Using in situ high-temperature atomic force microscopy during the solvent-free crystallization of an organic compound [Ni(quinolone-8-thiolate)₂], we observe long-range migration of NPs on a silica substrate and their incorporation into larger crystals, suggesting a non-classical pathway in the growth of the molecular crystal.