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Strong light–matter coupling within electromagnetic environments provides a promising path to modify and control chemical and physical processes. The origin of the enhancement of nonlinear optical processes such as second-harmonic and third-harmonic generation (SHG and THG) due to strong light–matter coupling is attributed to distinct physical effects, which questions the relevance of strong coupling in these processes. In this work, we leverage a first-principles approach to investigate the origins of the experimentally observed enhancement of resonant SHG and THG under strong light–matter coupling. For the proposed system under consideration, we find that the enhancement of the nonlinear conversion efficiency has its origins in a modification of the associated nonlinear optical susceptibilities as polaritonic resonances emerge in the nonlinear spectrum. Further, we find that the nonlinear conversion efficiency can be tuned by increasing the light–matter coupling strength. Finally, we provide an alternative framework to compute the harmonic generation spectra from the displacement field as opposed to the standard approach, which computes the harmonic spectrum from the matter-only induced polarization. Our results provide useful insights that shed light on this key debate in the field and pave the way for predicting and understanding quantum nonlinear optical phenomena in strongly coupled light–matter systems.