The water-processable microfibrillated cellulose (MFC) and sodium lignosulfonate (NaLS) system was used for the first time as carbon precursor. After drying and one-step pyrolysis up to 800 °C, the resulting bio carbonaceous char can acquire considerable electrical conductivity and porosity, making it a promising electrode material in energy storage devices. The present work characterizes MFC and NaLS separately as well as in blends in terms of morphology (SEM), structure (FTIR) and thermal stability (TG–MS). DTG patterns illustrate that MFC degrades in a narrow temperature range but at high reaction rates, whereas NaLS reacts more steadily in a larger temperature range. The thermal degradation rate of a MFC/NaLS blend differs from an additive weighted thermal degradation rate of each component: MFC and NaLS. The presence of sodium in the blends induced catalytic effects and made MFC decomposition to shift toward lower temperatures. This observation could be kinetically interpreted as a decrease of the MFC thermal degradation activation energies in the composites. The kinetics of MFC/NaLS blends is investigated by both model-free and mode-based methods, with the latter providing more appropriate Arrhenius parameters that allow reproducing the experimental curves