The electrode, where electrochemical reactions are taken place, plays a vital role in the overall performance of vanadium flow batteries (VFBs). In this paper, a composite of manganese oxide and amorphous carbon was first synthesized from metal-organic frameworks and used as a catalyst to promote the electrochemical behavior of electrodes. The newly proposed catalyst was fabricated by a carbonization process of Mn-based metal-organic framework obtained by hydrothermal method. The morphology, composition, electrochemical activity and cell performance are respectively studied. The results show that: i) the catalyst carbonized at 900 °C possesses a smaller size, larger specific surface area, more oxygen-containing functional group and realizes superior electrochemical activity; ii) it helps the vanadium flow battery to increase its energy efficiency by 6% at 100 mA cm−2; iii) it is also beneficial to the working voltage, discharge capacity, high current density property and cycling performance of VFBs. Thus, this work can not only provide a new type of electrochemical catalyst for vanadium flow batteries, but also be significant to the improvement of the overall performance and conducive to the widespread application of vanadium flow batteries.