Nanocomposites of nickel manganese layered double hydroxide (NiMn-LDH) and reduced graphene-oxide (rGO) with different loadings of rGO were synthesized by a simple solution-based in situ crystallization process. The synthesized composites were characterized by powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, field-emission scanning electron (FE-SEM) and high-resolution transmission electron (HR-TEM) microscopic techniques, energy dispersive X-ray analysis (EDX), and surface area and pore size analyses. The electrochemical activities of the NiMn-LDH/rGO nanocomposites were examined in aq. KOH in a three electrode assembly. It turned out that the composite of NiMn-LDH/rGO (NM/RG2) exhibited the highest electrochemical activity with excellent stability when compared to other compositions as well as pristine NiMn-LDH and rGO. The large surface area and enhanced conductivity due to the presence of the rGO in the NiMn-LDH/rGO composite are responsible for such high electrochemical activity. A solid-state hybrid energy-storage device consisting of NiMn-LDH/rGO composite as a positive electrode and rGO negative electrode exhibited the higher energy and power densities. The proto-type hybrid energy-storage device assembled using NiMn-LDH/rGO and rGO, was used to demonstrate the operation of a toy fan and a LED.