In the current study, a broad experimental program was designed and conducted to investigate the effect of aging conditions (i.e., aging temperature and time) on the thermomechanical properties (0.1% yield stress, pseudoelasticity, and recovery stress) of an FeMnSi-based shape memory alloy (FeMnSi-SMA) with a chemical composition of Fe-17Mn-5Si-10Cr-4Ni-1(V, C) (wt.%). In addition, transmission electron microscopy (TEM) was used to study the specimens' microstructures under several selected aging conditions. It was found that the aging conditions significantly affected the formation and distribution of precipitates and stacking faults in the aged specimens. At lower temperatures (e.g., 600 and 660 °C), numerous fine precipitates were randomly dispersed both in the austenite matrix and on/near stacking faults, resulting in higher yield stress and recovery stress. At elevated temperatures (e.g., 774 °C), the majority of the precipitates formed on/near stacking faults, leading to a lower yield stress and recovery stress. Besides, as the aging time increased, the pseudoelasticity increased at all aging temperatures.