The facile fabrication of proton exchange membrane (PEM) by intermixing of aminoethylpiperazine monomer (AEP), diamino diphenylmethane (DDM) monomer, and AEP combine with varying compositions of DDM (2, 4, and 6 wt%) copolymer with sulfonated polyethersulfone (SPES) and polyamide‐imide (PAI) polymer matrix for fuel cell applications. The intermixing of AEP@DDM copolymer in the SPES/PAI matrix was confirmed by Fourier transform infrared spectroscopy (FTIR) and X‐ray diffraction spectroscopy (XRD) techniques. The physicochemical performance of fabricated PEM was analyzed by water uptake, ion exchange capacity (IEC), swelling ratio, thermal degradation, hydrolytic degradation, chemical and mechanical properties. The intermixing copolymer (AEP@DDM) has significantly suppressed the thermal and hydrolytic deterioration of the PEM. Furthermore, the PAI had noteworthy to improve the thermal and chemical stability of the membranes significantly. The proton conductivity of the SPES/PAI/AEP@DDM (6%) membrane was found to be 5.68 × 10−4 S cm−1 which is 104% higher than the pristine SPES membrane. Besides, the selectivity of SPES/PAI/DDM (6%) was also enhanced up to 80.34% compared to pristine SPES membrane. The inclusion of AEP@DDM in SPES/PAI enhanced the proton conductivity and contributed to the membrane structure's uniformity, as revealed from morphological studies. The AEP@DDM copolymer modification of SPES/PAI membrane has shown good stability, physicochemical property and can be considered as a promising membrane material for fuel cell application.