Manganese dioxide and Ag-doped manganese dioxide powders were prepared by a chemical precipitation method using KBH4 as a reducing agent. The powders were studied by X-ray analysis, thermogravimetry, and electron microscopy. Composite electrodes for electrochemical supercapacitors (ES) were fabricated by impregnation of slurries of the precipitated powders and carbon black into porous nickel foam current collectors. In the composite electrodes, carbon black nanoparticles formed a secondary conductivity network within the nickel foam cells. Obtained composite electrodes, containing manganese dioxide and 20 wt% carbon black with total mass loading of 50 mg cm−2, showed a capacitive behavior in the 0.5 M Na2SO4 solutions. The capacitive behavior of the composite electrodes can be improved by mixing of manganese dioxide and carbon black in solutions or using Ag-doped manganese dioxide powders. The highest specific capacitance (SC) of 150 F g−1 was obtained at a scan rate of 2 mV s−1. The electrodes showed good cycling behavior with no loss in SC during 1,000 cycles.