In this Letter, we prepare FeNi-layered-double-hydroxide (LDH) nanoflakes and illustrate the corresponding microcosmic morphology. The optical response and laser modulation performance of the nanoflakes in the mid-infrared region were investigated for the first time, to the best of our knowledge. Employed as a saturable absorber in an ${\rm Er}{:}{\rm Lu}_2{\rm O}_3$ laser system, pulses with 256-ns duration were yielded at a repetition of 129 kHz under 7.8-W pump power, which corresponds to a pulse energy of 4.98 µJ.