Carbons were elaborated from purely lignocellulosic precursors (Microfibrillated cellulose and Lignosulfonates blends, simplified as MFC/LS blends) by slow pyrolysis (0.2 °C/min) in a large temperature range (400–1200 °C). They were characterized in terms of morphology (scanning electron microscopy), chemical functionalities (infrared spectroscopy), microstructure (Raman spectroscopy and X-ray diffraction) and physical properties (electrical conductivity and density evolution). MFC/LS carbons could achieve high electrical conductivity of 95 S/cm with regard to their low density, i.e.1.14 g/cm³ after pyrolysis at 1000 °C, compared to other biocarbons. The major aim of this work was to understand the electrical conductivity development in MFC/LS-derived biocarbons during the pyrolysis. A descriptive model, based on the progressive conversion of the biomass into conductive engineering carbons and composed of 3 distinct phases, was thus established to illustrate the electrical conductivity development phenomenon.