The physicochemical properties and proton conductivity are two important parameters of an effective polymer electrolyte membrane for a high-temperature fuel cell (HTPEMFC) (120 °C–180 °C). In this work, a novel composite membrane is prepared by poly (2, 5-Benzimidazole) (ABPBI) polymer matrix together with phosphonated multiwall carbon nanotube (PMWCNT) using the solvent casting method. The membrane typically exhibits fin-like projections due to the addition of PMWCNTs as characterised by SEM micrographs. The membrane also demonstrates enhanced proton conductivity and mechanical strength of 0.16 S cm−1 and 33 MPa respectively compared to pristine doped ABPBI membrane. Interestingly, the fabricated membrane is found to absorb the acid 2.15 times the pristine membrane whereas, acid leaching out per unit absorbed acid is reduced by 2.17 times compared to that of pristine membrane. Open circuit potential of 0.87 V with a fuel cell assembled with the composite membrane underlines better control on fuel crossover delivering a peak power density of 275.0 mW cm−2 compared to 212.8 mW cm−2 for the pristine membrane. Such enhancement in the power density (∼30%) is thus observed by tuning the polymer electrolyte hybrid nanocomposite membrane properties through functionalisation.