A general family of magnetic nanocomposites formed by FeNi3 ferromagnetic nanoparticles (NPs) embedded in a graphitized carbon matrix is reported. The soft chemical approach used relies on the catalytic effect of the NPs resulting from the thermal decomposition of the layered double hydroxide precursor, which acts as a multilayered nanoreactor enabling the formation of a range of carbon nanoforms (CNFs). This is followed by acid treatment of the as-prepared nanocomposites to isolate the different CNFs formed. These range from carbon nano-onions to graphene depending on the temperature of the thermal decomposition. This synthetic process paves the way for the rational design of metal–carbon nanocomposites with controllable composition as precursors of nanocarbons or even graphene. The coexistence of metal NPs and nanostructured carbon is a major source of applications. As a proof of concept, the electrochemical performance of these metal–carbon hybrid supercapacitors is studied under high discharging current densities and they exhibit high values of specific capacitance and excellent rate capabilities.