Anion exchange membranes (AEMs) have been recognized as one of the most prospective polyelectrolytes for fuel cells due to their faster electrode reaction kinetics and the potential of adopting cheaper metal catalysts against proton exchange membranes (PEMs). Herein, a series of poly(arylene ether sulfone)s containing a flexible pendant imidazolium cation were synthesized by grafting bromine-bearing imidazolium-based ionic liquids into a hydroxyl-bearing poly(ether sulfone) matrix. ¹H NMR spectroscopy was used to confirm the as-synthesized copolymers. Atomic force microscopy (AFM) and small angle X-ray scattering (SAXS) were used to characterize the morphology of the membranes. The incorporation of the flexible side-chain imidazolium groups is beneficial to the aggregation of the ionic clusters leading to the formation of hydrophilic/hydrophobic phase-separated morphology and nano-channels. As a result, an enhancement in the ionic conductivity can be achieved. Therefore, the as-prepared AEMs possess higher ionic conductivity than traditional benzyl-type AEMs. The weight-based ion exchange capacity (IEC_w) of the membranes was in the range of 1.01–1.90 meq. g⁻¹. Correspondingly, their ionic conductivity was in the range of 22.13–59.19 and 51.66–108.53 mS cm⁻¹ at 30 and 80 °C, respectively. Moreover, the membranes also exhibit good alkaline stability and interesting single cell performance. This work presents a facile and universal route for the synthesis of AEMs with superior performance.