The oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA) is one of the most attractive reactions to establish a sustainable chemical process based on biomass resources. In this work, a CNT-supported ruthenium–manganese oxide nano-catalyst (RuO₂/MnO₂/CNT) was employed for the electro-catalytic oxidation of HMF in base-free aqueous solution. The activity test showed that α-MnO₂ can effectively promote the activity of RuO₂ in the oxidation of HMF. In comparison with RuO₂/CNT, RuO₂/MnO₂/CNT possessed a lower activation energy and more than twice the FDCA formation rate. Under the optimized reaction conditions, the RuO₂/MnO₂/CNT catalyst afforded a FDCA yield of 72.1% in a 0.1 M K₂SO₄ aqueous solution at a 0.9 V (vs. Ag/AgCl) applied potential. To our knowledge, this is the first demonstration of FDCA formation as the primary product with high yield in an initially neutral electrolyte. The product FDCA can be easily separated after cooling the reaction solution to room temperature.