UNS S32101 lean duplex stainless steel (DSS) has a two phase microstructure consisting of austenite and ferrite in near equal proportions. Aging at high temperature results in formation of σ phase reducing ductility and also corresponding depletion of Cr from the matrix. The effect of long term aging on the microstructure and mechanical properties is still an active area of research for lean DSSs. In the present work effect of aging at 750 °C (up to 480 h) on microstructure and mechanical properties has been systematically studied. Scanning electron microscopy (SEM) revealed that the precipitates were distributed along ferrite/austenite (δ/γ) interfaces and ferrite/ferrite (δ/δ) boundaries. Nitrides (mainly Cr2N) were observed in initial phases of aging and σ phase afterwards, mostly at longer aging times. The decrease of Ni and Mo and increase in N in UNS S32101 steel delayed the precipitation of σ, but could not avoid it completely. The room temperature absorbed impact energy of specimens decreased gradually as the aging time increased. The lowest value of absorbed impact energy was found to be 24 ± 2 J after 480 h of aging, which was only ~11% as that of solution annealed specimen. Fractography also showed that the fracture morphology changed from fibrous (ductile) to dominant brittle (which involved predominance of cleavage facets along with long, wide cracks i.e. delamination fracture) with increasing aging time. Tensile tests also showed a decrease in ductility and an increase in yield stress/ultimate tensile strength with aging time.