<p>Equiatomic AlCoCrFeNi is a widely explored high-entropy alloy (HEA) because of its intriguing properties. Experimental literature states that AlCoCrFeNi would stabilize as either mixed phase (FCC, BCC and B2) or dominating FCC phase based on annealing conditions and the corresponding difference in atomic packing and microstructure which would substantially affect the overall properties of the alloy. In this regard, the present study uses atomistic simulations to investigate the evolution of microstructural, mechanical and thermodynamic properties of FCC phase-stabilized AlCoCrFeNi subjected to thermal treatment in the range of 298–2500&#xa0;K. Local lattice disorder and common neighbor analysis, grain segmentation and lattice dislocations analysis have confirmed the initiation of atomic displacements at 1500&#xa0;K. However, the complete phase transition was observed to happen in a broad temperature widow (1500–2200&#xa0;K) as confirmed from volume expansion ratio, coefficient of thermal expansion, radial distribution function and mechanical and thermodynamic properties. Abrupt changes in properties such as volume, density, elastic moduli, specific heat capacity, Debye temperature, sound velocity and hardness from 2200&#xa0;K confirmed the end point of phase transition. The broad window of thermomechanical stability was attributed to the sluggish atomic displacements till 1700&#xa0;K due to the presence of significant FCC phase concentration (~ 54%), which caused the occurrence of more number of slips, grain boundaries, slow atomic displacements and grain boundary diffusion. Further, the HEA showed excellent hardness (3.00&#xa0;GPa) and fracture toughness (2.25&#xa0;MPa&#xa0;m^0.5) even at 2200&#xa0;K and a low lattice thermal conductivity of 1.4&#xa0;W m⁻¹ K⁻¹ at 400&#xa0;K. Therefore, the present study provides comprehensive evaluation of the superior thermomechanical stability of FCC-stabilized AlCoCrFeNi.</p>