Ultrathin Au (or Ag) nanowires represent an excellent substrate for atomic layer deposition of Pt to afford highly active and cost-effective catalysts due to the large surface area and possible synergistic effect. An ideal synthesis of such nanowires should avoid using strong capping agents for convenient post-synthesis treatments and should be easily scaled up and reproduced in a high yield, which remains a challenge. Here, we report a novel strategy to synthesize sub-2 nm Au–Ag alloy nanowires with high quality in N,N-dimethyl formamide (DMF), which relies on Ag modification of the nanocrystal surface and Ag–halide interactions for regulating the one-dimensional growth of the nanowires, without involving strong capping agents that are usually required in conventional syntheses. Sub-monolayer Pt atoms were successfully deposited on these ultrathin Au–Ag alloy nanowires without forming ensembles despite a high loading amount (up to 20% in terms of Pt/(Au + Ag)) due to the large surface area. The resulting Au–Ag@Pt core/shell nanowires demonstrate superior activities in the formic acid oxidation reaction (FAOR) due to the synergistic ligand effect and the absence of Pt ensembles. We believe that the novel synthesis and the demonstration of these ultrathin Au–Ag alloy nanowires as a general platform for constructing cost-effective noble metal catalysts open new opportunities in designing catalysts for a broad range of reactions.