Rhombohedral shaped MnCO3 and its hybrid composite with in situ reduced graphene oxide (rGO) and multiwalled carbon nanotubes (MWCNT) have been synthesized by a simple and cost effective hydrothermal approach and examined as active material for supercapacitor application. The redox active MnCO3 showed high specific capacitance of 185F/g at 1.5A/g in 1M Na2SO4 electrolyte accompanied by high cycle stability. The high utility of the pseudocapacitive MnCO3 was achieved in its carbonaceous materials based hybrid composites exhibiting the maximum specific capacitance of 368F/g and 357F/g, respectively for the rGO-MnCO3 and MWCNT-MnCO3 composite at 1.5A/g current accompanied by very high cycle efficiency of 96% and 97% specific capacitance retention over 1000 charge discharge cycle at 3A/g. The high surface activity of the MnCO3 along with the synergistic effect with highly conductive and high surface area porous rGO and MWCNT resulted in their high energy density of 51.11Wh/kg and 49.58Wh/kg, respectively at a power delivery rate of 750W/kg suggesting both of them are competent to act as supercapacitor electrode material.