MLX19 stainless maraging steel grade exhibits a mechanical strength/fracture toughness balance within the required range for landing gear applications. However, the microstructure after the heat treatment still needs to be precisely controlled to obtain a better repeatability of the mechanical properties. This work shows that austenitizing is a critical stage. The influence of austenitization treatment parameters on the microstructure obtained after quenching was thus precisely quantified. It was first revealed that, after a standard austenitization at 850 °C and for specific heating rates and holding times, undissolved β-NiAl precipitates, reaching sizes up to 500 nm, still remain in the as-quenched state, in addition to a high retained austenite fraction. It was also found that large amounts of retained austenite are the result of local heterogeneities in the chemical composition of the austenitic phase prior to quenching, while the undissolved precipitates change the overall chemical composition of the austenitic matrix. New austenitization conditions were thus proposed, leading to a better homogeneity of the chemical composition of the martensitic matrix after quenching.