Martensitic steels have a wide range of mechanical applications due to their high strength. Martensite is a carbon-supersaturated iron with a body-centered tetragonal (BCT) structure. However, it is a metastable phase because it decomposes into carbon-free and carbon-rich nanodomains during aging, thereby changing its mechanical properties. To fully understand the mechanisms of this decomposition, thermodynamic and kinetic properties of carbon (C) atoms in BCT iron (Fe) are investigated by using Monte Carlo (MC) simulations. Pairwise interactions between carbon atoms are obtained by combining the linear elasticity theory and state-of-the-art density functional theory (DFT) results. This energy database is applied to MC simulations to predict the equilibrium carbon configuration and the collective kinetic motion of carbon atoms in an as-quenched martensite. From the metropolis MC simulation, we obtain a novel equilibrium phase of Fe₆C₂ structure. However, according to the kinetic MC simulations, it is difficult to reach this equilibrium phase during the martensite aging because, at room temperature or below, the carbon diffusivity is so slow that it will take an unrealistically long time for the system to achieve equilibrium. At a higher temperature, even though the kinetics are accelerated, the carbon concentration of the predicted equilibrium phase is so high that other metastable carbides can be formed before such equilibrium is reached. Moreover, the effects of the temperature, the applied stress, and the initial state on the aging kinetics are highlighted. The evolution of the carbon cluster concentration and the time scale of the aging kinetics are in good agreement with some existing experimental results.