Solid-state shear pulverization (SSSP) is an alternative polymer processing technique based on twin-screw extrusion with a continuous cooling system. In SSSP, low-temperature mechanochemistry modifies the macromolecular architecture and morphology, which in turn leads to physical property changes in the material. While a wide range of homopolymers, polymer blends, and polymer (nano)composites have been previously developed with SSSP, a fundamental understanding of how mechanochemistry affects polymer chain architecture and structure, and in turn, material properties, has not been elucidated. This paper conducts a systematic processing−structure−property relationship investigation of 10 thermoplastic polymers with varying properties, as they are subjected to consistent SSSP mechanochemical pulverization and nanocomposite compounding. Structural, mechanical, and thermal characteristics of the neat polymers are correlated to their response to SSSP by way of process covariants. Further, we investigate how SSSP processing parameters cause structural changes such as molecular weight reduction and filler dispersion level, which in turn dictate system properties like melt viscosity and thermal stability. Mechanochemical engagement with a high degree of physical contact during pulverization and compounding, characterized by the SSSP covariants exhibiting specific mechanical energy values above 4 kJ/g and an average screw temperature above 20 °C, is ensured when polymers have a glass transition temperature below the processing temperature (<50 °C) and high toughness (>40 MPa). Crystallinity and low thermal diffusivity (<0.2 mm 2 /s) are additional factors for engaged SSSP processing. Chain scission is an unavoidable outcome of SSSP, though the associated molecular weight reduction was <10% for 7 out of 10 polymers. The elucidated processing−structure−property relationships would allow the SSSP process for a given polymer system to be tailored to the specific needs for molecular structure alterations and performance improvements.