Fe-Mn-Al-C lightweight steels have great potential to replace Cr-Mo quenched and tempered steels where weight reduction is an important design variable. Previous research has centered around a fully austenitic cast composition of Fe-30Mn-9Al-0.9C-1Si-0.5Mo that achieves high strength through homogenous precipitation of κ-carbide during age hardening. However, the effect of varying Mo content and the corresponding effects on age hardening kinetics and resulting mechanical properties are not well documented. The current study investigates Mo additions in the range of 0-0.7% Mo on the microstructure, age hardening response, and toughness of a Fe-30Mn-9A-0.8C-1Si steel. Molybdenum was shown to decrease the age hardening response at 530 °C. Higher concentrations of Mo resulted in an increase in the Charpy V-notch impact toughness of aged specimens. Optical microscopy revealed that alloys with Mo additions had much less κ-carbide along grain boundaries in the aged condition and this decreased the propensity of intergranular fracture and increased toughness.