Organic polymeric microspheres grafted with ionic brushes are fillers incorporated into imidazolium functionalized polysulfone (ImPSF) forming a composite membrane. The ionic brushes create continuous and bulky hydrophilic phases in the ImPSF forming a composite membrane with the aggregation of hydrophilic groups at the brush/polymer interfaces. The hydrophilic phases promote ionic transport with decreased activation energies from 14.45 to 11.42 kJ mol−1 and increased effective ionic mobility from 0.84 × 10−4 to 2.19 × 10−4 cm2 s−1 V−1 compared to ImPSF with no brushes. The composite membrane shows increased hydroxide conductivities (up to 38.33 S cm−1 at 30 °C and 64.58 S cm−1 at 60 °C), which is 2.66 times and 2.01 times of the hydroxide conductivity of ImPSF membrane at the same conditions, respectively. Stability of the composite membrane is increased by replacing the imidazolium groups of polymer matrix and organic microsperes with more stable quaternary ammonium groups, such as N,N-dimethylhexylamine (DMHA). The residual hydroxide conductivity ratio of the composite membrane-DMHA is 83.4% after treating by 2.0 M aqueous NaOH at 80 °C for 96 h (vs. only 28.6% for the composite membrane with imidazolium groups under the same conduction). Meanwhile, the doubling increment of hydroxide conductivity in the composite membrane vs. the control membrane can be still retained (23.87 mS cm−1 for the composite membrane-DMHA vs. 8.97 mS cm−1 for the control membrane-DMHA).