Abstract: A lightweight Fe-26.9Mn-9.2Al-1C-2.7Ni high-entropy steel was fabricated via laser powder bed fusion (L-PBF) and subsequently subjected to a step-wise solid solution and aging treatment. This processing route effectively overcomes the strength-ductility trade-off, achieving an exceptional mechanical synergy, namely, the yield strength is enhanced by nearly 50% relative to the solid solution state, with a fracture elongation exceeding 50%. Analyses of the microstructural morphologies reveal that the submicron solute segregation networks, induced by the rapid non-equilibrium solidification during L-PBF, exhibit a pronounced chemical memory effect. These residual chemical fluctuations survive high-temperature annealing and precisely dictate the periodic precipitation of nanoscale B2 precipitates and ĸ-carbides along the original cellular boundaries during subsequent aging. Quantitative assessments confirm that this profound strengthening is primarily governed by the Orowan bypassing of dispersed B2 precipitates, coupled with a prolific multiplication of geometrically necessary dislocations (GNDs) at the phase interfaces. Concurrently, the activation of twinning-induced plasticity (TWIP) within the low-stacking-fault-energy austenite matrix synergizes with high-density dislocation networks to sustain a robust strain-hardening response, thereby suppressing premature plastic instability. The present work demonstrates that exploiting the substructure inheritance of L-PBF to tailor the unique microstructure offers a potential route to develop advanced additively manufactured lightweight structural materials.