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Understanding the nucleation and growth mechanisms of highentropy alloy (HEA) nanoparticles is crucial for developing functional nanocrystals with tailored properties. This study investigates the thermal decomposition of mixed metal salt precursors (Fe, Ni, Pt, Ir, Ru) on reduced graphene oxide (rGO) using in situ transmission electron microscopy (TEM) when heated to 1000 °C at both slow (20 °C min−1) and fast (103 °C s−1) heating/cooling rates. Slow heating to 1000 °C revealed the following: (1) The nanoparticles' nucleation occurred through multistage decomposition at lower temperatures (250−300 °C) than single metal salt precursors (300−450 °C). (2) Pt-dominant nanocrystals autocatalytically reduced other elements, leading to the formation of multimetallic FeNiPtIrRu nanoparticles. (3) At 1000 °C, the nanoparticles were single-phase with noble metals enriched compared to transition metals. (4) Slow cooling induced structural heterogeneity and phase segregation due to element diffusion and thermodynamic miscibility. (5) Adding polyvinylpyrrolidone (PVP) suppressed segregation, promoting HEA nanoparticle formation even during slow cooling by limiting atomic diffusion. Under fast heating/cooling, nanoparticles formed as a solid solution of fcc HEA, indicating kinetic control and limited atomic diffusion. The density function theory (DFT) calculations illustrate that the simultaneous presence of metal elements on rGO, as expected by the fast heating process, favors the formation of an fcc HEA structure, with strong interactions between HEA nanoparticles and rGO enhancing stability. This study provides insights into how heating rates and additives like PVP can control phase composition, chemical homogeneity, and stability, enabling the rational design of complex nanomaterials for catalytic, energy, and functional applications.