In the present work, two Fe-18Mn-10Al- xC low-density steels with different C concentrations were investigated. It is revealed that the difference in C concentration resulted in a marked difference in the microstructures of the investigated steels. The microstructure consisted of ferrite and austenite as well as of precipitates in Fe-18Mn-10Al-0.8C (0.8C steel); while carbides distributed on the austenite matrix in Fe-18Mn-10Al-1.2C (1.2C steel) after the same heat treatment. During deformation, dislocations exhibited a typical paired superdislocation feature in ferrite in 0.8C steel, extensive planar glide occurred in austenite in both steels and Taylor lattice-like structures, and well-developed microbands appeared in 1.2C steel. Overall, 0.8C steel demonstrated a tensile strength of 973.6 MPa and elongation of 44%, and 1.2C steel received a lower tensile strength (881.3 MPa) and a much higher elongation (78%). The strain hardening behavior of the two alloys was also different. The strain hardening rate decreased with strain in a non-monotonic feature for 0.8C steel, while it exhibited a moderate inflection in 1.2C steel.