We compare the structure and magnetic properties of mechanically alloyed Fe40Co30Ni30 medium-entropy alloy fabricated in argon (MA-Ar) and ambient (MA-Air) atmospheres. Irrespective of the atmosphere, the powders were micron-sized, constituting the nanocrystalline γ phase. The saturation magnetization (MS) of MA-Air was less than the MS of MA-Ar by a modest ~6%. The remanent magnetization (MR) and intrinsic coercivity (HCI) of MA-Air were higher than MA-Ar by ~19% and ~28%, respectively. At below-ambient temperatures, both MA-Ar and MA-Air exhibit similar behavior, i.e., MS, and HCI increased with a decrease in temperature. The thermomagnetic behavior of MA-Ar and MA-Air indicated the phase transformation γ α (Fe) + γ (Fe deplete) at above-ambient temperatures, between ~725 K and ~800 K. The Range of Value Method utility function analyses assigned MA-Air the highest utility value, among the as-prepared and thermally-treated Fe40Co30Ni30 alloy powders, suitable as a ferromagnetic precursor to fabricate semi-hard magnets.