In this work, Cladophora glomerata, a harmful seaweed, is converted into an olive-shaped magnetic biochar by a slow pyrolysis process catalyzed by iron. The resultant magnetic biochar has a high surface area of 296.4 m2 g−1 with a carbon-rich structure that makes it suitable to be used as an electrode in Li-ion batteries. The catalytic pyrolysis process showed significant effect on steam reforming, ketonization and deoxygenation and/or denitrogenation reactions. The overall quality of the pyrolysis products increases: the gas contains a higher percentage of hydrogen (up to 22%), while the oil is enriched in furans (with a selectivity of about 14%). The electrochemistry behavior of magnetic biochar has been also evaluated, using galvanostatic charge–discharge, cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS) analyses. The electrochemical results indicated a higher initial specific discharge capacity (740 mAh g−1) and great cyclic stability for magnetic electrode as compared to the biochar electrode.