Oxidative coupling of methane (OCM) is a promising process for the single-step conversion of CH4, the major component of natural gas (NG), to value-added C2 products. Among hundreds of alkali, alkaline earth and transition metal and metal oxide-based bulk and supported catalysts tested for this reaction, the supported Mn-Na2WO4/SiO2 catalysts have been found very promising due to their excellent thermal and chemical stability (up to 1000 hours on stream) and high C2 product yield (~25%). Despite large numbers of structural and kinetic investigations on this catalyst system, the stable structure, nature of the active site, and reaction mechanism are still under debate due to lack of in-situ/operando characterization and time-resolved chemical probing. These discrepancies have hindered the improvement of this catalyst system for practical implementation in NG utilization.The objective of the current study is two-fold: (i) to establish molecular level structural insights of supported Mn-Na2WO4/SiO2 catalyst structure during the OCM reaction, and (ii) to decipher the complex OCM reaction mechanism over this catalyst with the aid of transient, highly time-resolved chemical characterization and steady-state kinetic studies. The state-of-the-art in-situ Raman, UV-Vis, XRD, and NAP-XPS studies reveal, for the first time, that the freshly calcined supported Mn-Na2WO4/SiO2 catalysts possess surface Na-WOx and MnOx species along with crystalline phases of Na2WO4, Mn2O3 and cristobalite SiO2 support. During the OCM reaction, the crystalline Na2WO4, Mn2O3 phases were unstable due to melting and reduction, respectively, whereas the surface Na-WOx and MnOx species are both thermally and chemically stable. Further investigations via transient analysis of products (TAP) technique reveal that the supported Na2WO4/SiO2 catalysts possess two different types of lattice oxygen species at OCM relevant temperature: (i) molecular O2* type species originate from the lattice of molten Na2WO4 phase, and (ii) atomic O* type is associated with surface Na-WOx sites. Advanced kinetic investigations demonstrate that (i) surface Na-WOx sites are responsible for selectively activating CH4 to C2Hx and over-oxidizing CHy to CO, and (ii) molten Na2WO4 phase is mainly responsible for over-oxidation of CH4 to CO2 and also assists in oxidative dehydrogenation of C2H6 to C2H4. Finally, the promotion mechanism of Mn was deciphered to show that the Mn-oxide phase, in itself (Mn/SiO2 catalyst), is less active and highly unselective towards CH4 oxidation. However, Mn addition (i) improves the reducibility and release rate of lattice oxygen species, (ii) helps in low-temperature activation of lattice oxygen species, and (iii) improves the exchange of gas-phase oxygen and lattice oxygen species present in supported Na2WO4/SiO2 catalysts. These promotional effects of Mn are reflected in higher CH4 activity and C2H4 to C2H6 ratio during steady-state OCM reaction. These new findings will guide the rational design of catalysts with higher activity and product selectivity.