Despite the Minamata Convention's efforts to reduce mercury in industry, acetylene hydrochlorination still relies on a highly toxic HgCl₂ catalyst, forcing the search for environmentally benign catalysts. The targeted development of new catalytic systems is hampered by the lack of well-defined concepts about the mechanism of the heterogeneously catalyzed reaction. Herein, density functional modeling of the energy profile for catalytic acetylene hydrochlorination over mechanically preactivated K₂PtCl₄ salt was correlated with experimental data, confirming the concept of "noninnocent/cooperative" ligands. Chloroplatination of π-coordinated acetylene by the HCl molecule to form the β-chlorovinyl Pt^II derivative is shown to be the rate-limiting step. It involves the attack by the Cl atom of the HCl molecule on the C atom of the π-complex to form the product of exclusively trans(anti)-addition across the triple bond, in agreement with the experiment. The neighboring platinum complex facilitates this conversion by donating a chloride ligand to bind the released H⁺ to form a new HCl molecule. In addition, the HCl/DCl kinetic isotope effect (KIE) estimated from the calculations is 3.1, close to KIE = 3.7 determined experimentally. These results establish a mechanistic understanding of the acetylene hydrochlorination over K₂PtCl₄ and highlight the involvement of two Pt-cites in catalyzing the reaction.