The formation of dimethyl carbonate (DMC) from CO2 and methanol with the dimer [n-Bu2Sn(OCH3)2]2 was investigated by experimental kinetics in support of DFT calculations. Under the reaction conditions (357–423 K, 10–20 MPa), identical initial rates are observed with three different reacting mixtures, CO2/toluene, supercritical CO2, and CO2/methanol, and are consistent with the formation of monomeric di-n-butyltin(IV) species. An intramolecular mechanism is, therefore, proposed with an Arrhenius activation energy amounting to 104 ± 10 kJ mol−1 for DMC synthesis. DFT calculations on the [(CH3)2Sn(OCH3)2]2/CO2 system show that the exothermic insertion of CO2 into the Sn–OCH3 bond occurs by a concerted Lewis acid–base interaction involving the tin center and the oxygen atom of the methoxy ligand. The Gibbs energy diagrams highlight that, under the reaction conditions, the dimer–monomer equilibrium is significantly shifted towards monomeric species, in agreement with the experimental kinetics. Importantly, the two Sn–OCH3 bonds are prompt to insert CO2. These results provide new insight into the reaction mechanism and catalyst design to enhance the turnover numbers.