We combined ab initio molecular electronic Hamiltonians with a cavity quantum electrodynamics model for dissipative photonic modes and applied mean-field theories to the ground-and excited-states of resulting polaritonic systems. In particular, we developed a non-Hermitian configuration interaction singles theory for mean-field ground-and excitedstates of the molecular system strongly interacting with a photonic mode, and applied these methods to elucidating the phenomenology of paradigmatic polaritonic systems including the ground-state polaritonic structure of formaldehyde coupled to cavity modes that were shown to modify the symmetry of the ground-state wavefunction, and the polaritonic potential energy surfaces of the magnesium hydride ion coupled to a lossy photonic mode. A reference implementation of this method and the CQED-RHF method was realized using the Psi4Numpy framework which can be accessed using links provided within the text.