High entropy alloys (HEAs) are an important new material class with significant application potential incatalysis and electrocatalysis. The entropy-driven formation of HEA materials requires high temperaturesand controlled cooling rates. However, catalysts in general also require highly dispersed materials, i.e.,nanoparticles. Only then a favorable utilization of the expensive raw materials can be achieved. Severalrecently reported HEA nanoparticle synthesis strategies, therefore, avoid the high-temperature regime toprevent particle growth. In our work, we investigate a system of five noble metal single-source precursorswith superior catalytic activity for the oxygen reduction reaction. Combining in situ X-ray powderdiffraction with multi-edge X-ray absorption spectroscopy, we address the fundamental question of howsingle-phase HEA nanoparticles can form at low temperatures. It is demonstrated that the formation ofHEA nanoparticles is governed by stochastic principles and the inhibition of precursor mobility during theformation process favors the formation of a single phase. The proposed formation principle is supported bysimulations of the nanoparticle formation in a randomized process, rationalizing the experimentally founddifferences between two-element and multi-element metal precursor mixtures.