The thermal degradation kinetics and mechanisms of thermosetting resins based on 4-(N-maleimido)phenylglycidylether (MPGE) were investigated with TGA, GC/MS, and FTIR. Two samples from MPGE cured with 4,4′-diaminodiphenylmethane (DDM) and diethylphosphite (DEP), respectively, were studied. The MPGE based resins showed good thermal stability over conventional epoxy resins. The degradation of MPGE–DEP was found to be initially triggered at the phosphonic ester linkage, while MPGE–DDM reacted at the epoxy linkage. Introducing phosphorus therefore significantly altered the patterns, activation energies, and mechanisms of the degradation reactions of the MPGE resins. This change caused the MPGE–DEP resin to show good thermal stability, high activation energies of degradation, and enriched char formation.