Ammonia-2-methylimidazole (2MI) assisted MnOx nanocube were deposited on Mn-2MI polyacrylonitile (PAN) electrospun nanofibers through wet-impregnation. Subsequent annealing under argon atmosphere resulted in formation of MnOx–carbon nanofiber (CNF) composite. The 2MI-derived carbon matrix around the MnOx nanocubes provided faster electron transport and efficient electrochemical performance. The optimal MnOx-CNF depicted areal capacitance of ~ 2.4 F·cm−2 at current density of 0.75 mA·cm−2. Long-term charge-discharge cycling at 1 mA·cm−2 shows a capacitance retention of 97% after 15,000 cycles. The extended stable potential window of 1.6 V provided energy and power density of 382 µW· cm−2 and 16 mW· cm−2. The findings revealed that the ammonia-2MI assisted MnOx nanocube-CNF as robust electrodes for supercapacitors.