We theoretically show how collective quantities, such as the Rabi splitting Ω R , manifest in the change of the PMET rate constant by rigorously deriving a Marcus-type rate constant. This rate theory suggests a possible mechanism that only depends on the Ω R = 2 √ Ng c , even though the light-matter coupling strength per emitter g c is small. We numerically verify the validity of this rate constant expression using the numerical simulations. Further, by driving the UP state with a CW laser in a positively detuned cavity, the effective PMET rate constant can be several orders of magnitude larger than outside the cavity even with significant molecular disorder (that causes transition to the dark states) and cavity loss (that causes population decay from UP to the ground state). These results reveal a promising mechanism to realize photochemical rate enhancement with collective strong coupling in disordered and lossy polariton systems as well as enable otherwise impractical uphill ET reactions. More importantly, our work demonstrate a promising principle on how delocalized light-matter interactions between one cavity mode and N molecules can induce changes of the rate constant for local chemical transformation.