Motivated by the use of frontal polymerization for the manufacturing of thin layers with thickness down to ∼100 μm, we numerically and experimentally investigate the interplay between the exothermic polymerization and the heat losses to the substrate. The results show how the reaction–diffusion power balance affects the velocity and temperature of the polymerization front. The lower thickness limit of the thin layer depends on the cure kinetics of the resin and the thermal properties of the substrate and is described by a scaling law. Concurrently, thin-layer frontal polymerization experiments confirm the effects of front–substrate interaction on the front propagation and the limiting thickness of the polymerized layer. The present study offers a fundamental understanding of the resin–substrate interplay in thin-layer frontal polymerization and provides a quantitative description of the limiting thickness of the layer in the fabrication of functional thin polymeric parts and resin coatings.