In this letter, we theoretically demonstrate that the coherence lifetime between the upper and lower polariton states in the collective coupling regime increases with an increasing collective Rabi splitting 2 √ N g c . This is confirmed by computing ρ +-(t) using exact quantum dynamics simulation through the HEOM approach, as well as through analytic rate theory using Fermi's Golden Rule. We found that the main mechanism for decoherence under this collective coupling regime at resonance condition largely comes from population transfer from the upper polariton state to the dark states manifold, a departure from the pure dephasing limit that does not involve any population transfer. Using analytic theory based on FGR expression, we showed that polariton decoherence can be mitigated by reducing excitonphonon couplings. An enlarged energy gap between the polariton states and the dark states further reduces the population relaxation rate from |+⟩ to the dark state manifold, as well as the decoherence rate. Further, we showed that this enhancement in coherence is robust even in the presence of cavity loss, with a range of quality factors that can be achieved using the state-of-theart FP cavities. By investigating the coherence enhancements with varying light-matter detunings, we further demonstrated the importance of the dark states in mediating the coherences between the polaritonic states and theoretically predicted and explained the turnover in T 2 for positive ∆ as a consequence of competition between transitions from |+⟩ → {|D k ⟩} and |-⟩ → {|D k ⟩}. The results in this letter use parameters that are representative of those used in recent polariton experiments using CdSe NPL coupled to DBR FP cavities. We thus expect that these theoretical predictions can be directly verified experimentally and will provide crucial insights into understanding polariton 2D spectroscopy data. Supporting Information. The Supporting Information is available free of charge at [url]. Details of the HTC system in the polaritonic basis; FGR rates for polariton transitions; details of HEOM simulations; purity results for a fixed number of molecules; coherence data for a fixed number of molecules in a lossy cavity; coherence data for single molecule case (N = 1); discussion of Gaussian to Markovian transition in the collective coupling regime; FGR analysis of the decoherence turnover with increasing positive detunings.