Invar alloys have been widely used recently due to their extremely low coefficients of thermal expansion (CTE). The development of thin Invar alloy foils is essential for widespread use in the industry such as a shadow mask, flexible display, and wearable devices. Traditional Invar alloy fabrication process like rolling has fatal disadvantages in obtaining thin films. Electroforming has emerged as an important approach to overcome the current problem in terms of cost and thickness. However, till date, electroformed Invar alloys have BCC–FCC mixed phases and relatively high CTE due to nanoscale grain size. In this study, we incorporate Mn to the nanograin Invar alloy foils by electroforming process to stabilize the FCC phase, assisted by thermodynamic expectation using FactSage. As the amount of Mn increases, the CTE decreases. Although electroformed Fe–Ni–Mn alloys have a relatively high CTE than conventional Invar alloys because it still has nanograins, the CTE could be further reduced by recrystallization process. Our results show that the incorporation of Mn into Invar alloys can stabilize the FCC phase near the room temperature, and electroforming is promising to replace the conventional rolling process for thin metal foils.