Crystallization in polymers provides a fundamental probe of how molecular relaxation governs the emergence of order in glass-forming systems. Despite the longstanding success of the Lauritzen–Hoffman (L–H) theory, its empirical treatment of molecular diffusion fails to capture the crystallization kinetics observed in the deeply supercooled regime. Here, we establish a physically grounded framework that replaces the empirical mobility in the L–H formalism with a weighted combination of two measurable relaxation modes, namely the structural α-relaxation, governing cooperative dynamics, and the slow Arrhenius process (SAP), associated with localized molecular rearrangements. This unified two-channel description quantitatively reproduces crystallization kinetics over several decades in time across twelve polymers spanning a wide range of fragility. In addition, our framework remains predictive under nanoconfinement, where interfaces and finite-size effects modify both mobility and nucleation. The results reveal that polymer crystallization proceeds through coupled long-range transport and localized ordering events governed by the same relaxation hierarchy that underlies vitrification, thus bridging glassy dynamics and crystallization kinetics at the molecular level.