This effort provides a multifaceted analysis of the structural changes and material dynamics of thermally driven softening and curing of three distinct phthalonitrile (PN) resins that cross‐link into thermally stable and oxidation‐resistant thermosets. Although this material system has yielded a large subset of fire‐retardant composites that require facile processing and low‐temperature curing, to date, insufficient information had been available on the fundamental processes that drive their softening and curing stages. Our approach conducted a complementary analysis the chemistry, monomer mobility, and rheology of three PN polymers in order to correlate the curing processes with corresponding structural and behavioral transformations of thermosets. We focused on PNs with a bisphenol S backbone, a bisphenol A (PEEKTM‐like) backbone, and a resveratrol backbone. We relied on quasi‐elastic neutron scattering (QENS) in order to analyze the in situ dynamics and self‐diffusion properties of PN monomers, and to track changes in their mobilities during cross‐linking and staging. Our analysis facilitates proper control over the staging and final curing of these resins and enables more efficient processing of these thermosets.