Atomic layer deposition (ALD) of tungsten nitride (WN) enhanced the durability and activity of a carbon-supported Pt nanoparticle oxygen reduction reaction (ORR) catalyst by adding protective WN nanostructures to the surface. A post-synthesis thermal treatment sequence of low-temperature oxidation followed by high-temperature reduction was carried out on the ALD-modified catalyst. The effects of the ALD process and thermal treatment on the structure and electrochemical properties of the catalyst were examined. Characterization through ICP-MS, STEM and EDS mapping, and X-ray diffraction showed that the ALD process deposited WN homogenously across the surface. Thermal treatment resulted in a reduced amount of nitride on the surface and produced separate Pt and W domains. The electrochemical performance of the catalysts is measured through rotating disk electrode voltammetry. The thermally-treated ALD catalyst was observed to have a high mass activity towards the ORR (465 mA/mg), surpassing benchmark Pt/C (277 mA/mg), and demonstrated superior retention of electrochemical properties after an accelerated durability test. Analysis of particle size distributions before and after durability testing also indicated that the mechanical stability of the Pt nanoparticles is enhanced in the thermally-treated ALD catalyst compared to the uncoated materials.