Magnetic nanoparticles due to their high surface-to-volume ratio play vital roles in diverse applications, including tunable optical, electrical, catalytic, biological, nanofluidic, electrochemical supercapacitor, and data storage. However, there is a pressing need for proper surface functionalization of such nanoparticles in achieving the desired material properties for multi-functional applications. In this regard, aryl diazonium salts are one of the intriguing components to modify the surfaces of NiFe₂O₄ nanoparticles through a radical mechanism forming robust covalent bonds, which create stable interfaces at the NP-organic molecules. Complimentary surface analysis techniques are utilized to ensure the formation of NPs and covalent surface functionalization. An enhancement of saturation magnetization in the functionalized NPs was observed as compared to the pristine NiFe₂O₄ NPs ensuring excellent nexus between NPs and organic molecules. NiFe₂O₄, a heterometallic system containing electroactive metal ions of different charges can impact the overall capacitance. A sharp increase in specific capacitance was recorded in 3 M KOH (∼1279 F g⁻¹) of the modified NiFe₂O₄ NPs compared to the unmodified NPs along with high retention of capacitance (∼82–90%) after 2000 cycles. This high specific capacitance originates from the larger conduction channels to the electrolyte ions in aryl-modified NiFe₂O₄ NPs compared to pristine NPs. These results open very promising avenues to design surface-functionalized magnetic NPs for future energy storage applications.