Determining the formation and growth mechanism of bimetallic nanoparticles (NPs) with atomic detail is fundamental to synthesize efficient “catalysts by design”. However, an understanding of the elementary steps which take place during their synthesis remains elusive. Herein, we have exploited scanning transmission electron microscopy coupled to energy-dispersive X-ray spectroscopy, operando wide angle and small-angle X-ray scattering, and electrochemistry to unveil the formation and growth mechanism of hollow PtNi/C NPs. Such NPs, composed of a PtNi shell surrounding a nanoscale void, catalyze efficiently and sustainably the oxygen reduction reaction (ORR) in an acidic electrolyte. Our step-by-step study reveals that (i) Ni-rich/C NPs form first, before being embedded in a Ni_xB_yO_z shell, (ii) the combined action of galvanic displacement and the nanoscale Kirkendall effect then results in the sequential formation of Ni-rich core@Pt-rich/C shell and ultimately hollow PtNi/C NPs. The electrocatalytic properties for the ORR and the stability of the different synthesis intermediates were tested and structure–activity−stability relationships established both in acidic and alkaline electrolytes. Beyond its interest for the ORR electrocatalysis, this study also presents a methodology that is capable to unravel the formation and growth mechanism of various nanomaterials including preferentially shaped metal NPs, core@shell NPs, onion-like NPs, Janus NPs, or a combination of several of these structures.