This paper describes the derivation of boron-manganese-carbon nanocomposites by CO2 carbonization using sodium borohydride (NaBH4) as a reduction agent at 1 bar, followed by impregnation of boron-doped porous carbon (BPC) with a form of manganese oxide (MO). The prepared composites (BPCMO) can be used as an advanced electrochemical energy material, such as active electrocatalysts for oxygen reduction reaction (ORR) and as electrode materials for supercapacitors. Various spectroscopic and microscopic measurements were carried out to investigate the morphology and structure of the BPCMO. Among many types of manganese oxide, it was confirmed that only Mn3O4 was embedded in the BPC. Cyclic and linear sweep voltammetry indicated that the BPCMO exhibits a four electron transfer pathway and has electrocatalytic activity comparable to that of commercial Pt/C. Galvanostatic charge/discharge and electrochemical impedance spectroscopic measurements showed that the BPCMO provided remarkable capacitance (150 F g(-1) at 1.0 A g(-1) and 136 F g(-1) at 10.0 A g(-1)) compared to that of BPC (58 F g(-1) at 1.0 A g(-1) and 15 F g(-1) at 10.0 A g(-1)), with a highly stable capacitance retention of 93.9% over 3500 charge/discharge cycles. It was found that impregnation of BPC with Mn3O4 enhanced electrochemical performance by generation of new active sites, increase in specific surface areas, and reduction of overall resistance.