Transformation-induced plasticity (TRIP) steels from the third generation of advanced high-strength steels (AHSS) contain Si additions to prevent the formation of carbides. Cementite (Fe3C) is a prototype among the carbides, and despite the importance of the influence of alloying elements on its stability, mechanisms by which the elements act have not been clarified so far. Herein, ab initio calculations are employed to study the impact of several alloying elements, including Al, Cr, Mg, Mn, and Si, on the stability of cementite. Partitioning energies are calculated to determine the segregation tendency of alloying elements between the phases such as ferrite, austenite, and cementite. The change in formation energy between the alloyed cementite and the pure cementite is then used to quantify the phase (de)stabilization. Therefore, both the partitioning energy and the change in formation energy must be considered together in a multiphase alloy system to make statements about the effect of alloying elements on the cementite stability are proposed. In addition, the effects of the technically most important elements Al and Si on the mechanical properties of cementite are calculated using the stress–strain method. Both the elements are found to increase the elastic stability of cementite.