For decades, it has been recognized that alloying platinum (Pt) with a secondary metal can enhance the catalytic activity of the oxygen reduction reaction (ORR) compared to pristine Pt catalysts. However, the mechanisms underlying this phenomenon vary significantly from one alloy to another. Here, we report the results of a computational study on the origin of the experimentally observed enhanced ORR activity of Ag_xPt_1-x/Pt(1 1 1) monolayer surface alloy with 7 %–50 % Ag contents. A phase-separation model was developed and able to generate 2D phase-separation distributions of Ag and Pt atoms in Ag_xPt_1-x/Pt(1 1 1) surfaces in line with atomic resolution scanning tunneling microscopy. We employed DFT-calculated *OH adsorption energy as a descriptor to obtain the activity of those surfaces, which reveals the ORR activity dominated by the reaction on Pt(Pt₆) heptamers and also gives evidence of long-range self-induced surface strain as the source of the enhanced activity of binary Ag_xPt_1-x/Pt(1 1 1) surfaces, i.e., the slightly larger surface Ag atoms induce a compressive strain of Pt-Pt bonds of the Pt(Pt₆) heptamers, which increases the activity of binary surfaces compared to the pristine Pt(1 1 1) surface. Moreover, the excellent simulated-experimental agreement for the polarization curves shows the high quality of this approach and its more general potential for an improved understanding of the catalytic properties of inhomogeneous binary surfaces as the basis for a rational design of binary catalysts.