Understanding and control of crystallographic polymorphism and crystal habit of organic compounds is scientifically and technologically important to several industries. Since a polymorph is determined at the early stages in crystallization, methods that lead to an advanced understanding of early crystal formation pathways and mechanisms are highly desirable. In this work, we have introduced time-resolved in situ wide-angle X-ray scattering (WAXS) at Cornell’s High Energy Synchrotron Source (CHESS) to study the early formation stages of the Form I and II crystallization events of a pharmaceutical compound, acetaminophen (ACM). Studies were informed by results from earlier investigations that both the surface chemistry of self-assembled monolayers (SAMs) as well as solvent conditions work together to control crystal polymorph. Studying crystallization of Form II by seeded nucleation, we verified that crystals grow faster at the substrate-solution interface than in the bulk above, and that the PTS (trichloro(phenyl) silane) SAM surface is taking over in controlling crystal growth by directing the (002) planes from slightly out-of-plane to a totally in-plane orientation. Studying crystallization of Form I by spontaneous nucleation, we identified unusual shifts along the scattering vector, q, of the earliest peak occurring in the in-situ scattering patterns. Further analysis and corroboration by other data sets pointed to the possible existence of structural transformations at these early cryastal formation stages. Our results indicate that our methodologies are effective to gain insights into the earliest formation stages of the crystallization of ACM and are now being extended to other model compounds.