Mesopore-enriched activated carbon nanofiber mats incorporating RuO2 (RuPM-ACNF) are prepared by a simple electrospinning method with the help of poly(methyl methacrylate) (PMMA), and their electrochemical property electrodes are investigated as a supercapacitor electrode. The microstructure of RuPM-ACNF is changed in terms of high specific surface area, narrow pore size distribution, and tunable porosity. The textural parameters affect the electrochemical properties significantly through the variation in PMMA concentration. The active sites, RuO2 and abundant mesopore structures of RuPM-ACNF not only increase the specific capacitance but also enhance the high-rate electrochemical performance by fast electrolyte ion diffusion into the pores with increasing PMMA concentration. The RuPM-ACNF electrode also exhibits good cycling stability after 3000 charge–discharge cycles, which demonstrates the good stability, long lifetime and high degree of reversibility in repetitive charge–discharge cycling of the RuPM-ACNF electrodes. Therefore, the cooperation of large mesopores induced by PMMA and RuO2 in the ACNF electrode materials synergistically improves the electrochemical performance because of the electric double-layer capacitance of porous ACNFs and the pseudocapacitance of RuO2, resulting in high rate capability, high capacitance, and long cycling life.