Strongly coupled light–matter systems are becoming a ubiquitous platform for investigating an increasing number of physical phenomena from modifying charge transport, altered emission, and relaxation pathways to selective or enhanced chemical reactivity. Such systems are investigated across a large length scale from few-nanometer-sized particles to macroscopic cavities encompassing many interacting moieties. Describing these numerous and varied physical systems is attempted in various ways from classical coupled harmonic oscillator models through quantum Hamiltonians to ab initio modeling. Here, by combining time-dependent density functional theory modeling and analysis with macroscopic models, we elucidate the origin of modifications of effective interaction parameters in terms of microscopic changes to the electronic density and Kohn–Sham transitions of the plasmonic particle and its coupled molecular counterpart. Specifically, we demonstrate how the emergence of mixed metal-molecular states and transitions modifies the effective resonances of the underlying plasmon and molecule in the regime of strong coupling and how these changes subsequently lead to the formation of mixed light–matter polaritons.