We study the Fano effect in dissipative cavity quantum electrodynamics (QED), which originates from the interference between the emitter's direct radiation and that mediated by a cavity mode. Starting from a two-level system coupled to a structured reservoir, we show that a quantum master equation previously derived within the Born-Markov approximation can be rederived by introducing a single auxiliary mode through the pseudomode approach. We identify the corresponding spectral function of the system-environment interaction and show that it consists of a constant contribution and a non-Lorentzian contribution, whose interplay gives rise to a spectral profile of the Fano form. The constant contribution represents a Markovian background and is essential for obtaining a Lindblad master equation. Furthermore, by applying Fano diagonalization to an atom-cavity system coupled to common and independent reservoirs, we independently derive the same spectral function and clarify its physical origin. Our results provide a unified description of the Fano effect in single-mode cavity QED systems and reveal its non-Markovian origin encoded in the spectral function of the structured reservoir.

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• Cavity quantum electrodynamics
• Fano resonance