In this study, FeCrNiCoMo maraging stainless steel was fabricated by laser melting deposition. The initial nonequilibrium microstructure exhibited cellular segregation of Cr and Mo with a 10–20 μm cell size. We investigated the microstructural evolution, particularly during aging with different time(1 h, 3 h, 5 h), for three post heat treatments: direct aging (500 °C) and low-temperature solution treatment (750 °C) and aging without and with additional prior homogenization (1000 °C). Microsegregation profoundly affected the austenite reversion behavior. The segregation at the cell walls promoted the nucleation and growth of reverted austenite, which eventually linked to form a net-like structure after prolonged aging. The net-like austenite in the direct-aged samples exhibited low thermal stability and result in brittle fracture upon impact at the temperature of liquid nitrogen (−196 °C). Net-like austenite and abundant lath austenite in the cell interior were found in the low-temperature solution-treated samples, which exhibited well-matched strength, toughness, and annealing temperature. Prior homogenization eliminated the microsegregation-induced net-like austenite. The measured mechanical properties indicate that the segregation-induced net-like austenite failed to benefit both the strength and toughness of the studied steel.