We report an ordered Pt2CoNi ternary alloy nanoelectrocatalyst, synthesized via simple molten salt synthesis (MSS) procedure and also defined theoretically a new descriptor to explain the origin of exceptional oxygen reduction reaction (ORR) activity. The catalyst consists of a very thin layer of carbon, since the seed-growth of such ordered Pt2CoNi nanoelectrocatalyst has been originated through the pores of high surface area carbon during a MSS. Electrocatalytic ORR activity of Pt2CoNi nanoelectrocatalyst is 5–6 times higher than that of commercial Pt/C catalyst with fascinating stability behavior in acidic media. Most interesting behavior of this Pt2CoNi has been observed after 15,000 and 25,000 durability cycles, where 16 times activity enhancement is achieved at 0.9 V. Furthermore, after 25,000 cycles, a specific activity of 0.605 mA/cm2Pt (22 times higher than its own) has been observed at 1.0 V for the first time. Density functional theory (DFT) based calculation is used to estimate the onset potential by plotting the free energy profile for ORR of each active site of nanocatalyst. New descriptors are proposed to explain the origin of exceptional catalytic activity of ternary metal nanoelectrocatalyst.