Herein, a surfactant-free, ethylene glycol-mediated synthesis of PtIr nanoalloys was optimized. In particular, a post-synthesis treatment was identified as the key step in order to determine the nanoparticles size and their organization in the nanostructure, depending on the presence of a reducing agent and on pressure conditions. After synthesis, the as-obtained nanomaterials were broadly characterized: SEM and TEM images, EDX maps and XRD spectra showed the formation of nanorods with a few nanometers size and similar quantitative compositions of platinum and iridium. Afterward, the electrocatalytic activity towards the methanol oxidation reaction of the synthesized nanomaterials was tested and the best sample, treated under a hydrogen/nitrogen flow at 10 bar, exhibits a negligible onset potential (0.058 V) and a very high If/Ib ratio (2.5). Moreover, the aforementioned sample was tested as an electrochemical sensor for the detection of small traces of ammonia in an aqueous solution with a limit of detection of 4.88 μM. The sensor was tested also in simulated wastewater coming from the fertilizer industry, showing proper operation and excellent selectivity.