We report self-assembly of a statistical copolymer poly(styrene-co-methyl methacrylate) (P(S-co-MMA)) containing ionic liquid (IL)-philic methyl methacrylate (MMA) and IL-phobic styrene (S) repeating units in IL for fabrication of electrolyte-gated organic transistors. P(S-co-MMA)s with high MMA contents were synthesized by copolymerization of styrene and MMA via a reversible addition–fragmentation chain transfer (RAFT) process, and their behavior in 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]) was investigated. While dynamic light scattering analysis showed formation of a micellar solution at low concentration, the elastic modulus of the viscoelastic solution increased significantly more than the loss modulus at high concentration. Small angle X-ray scattering analysis suggested ill-defined phase separation between PS-rich segments and PS-lean segments swollen in [EMI][TFSI]. The resulting P(S-co-MMA)/[EMI][TFSI] mixture exhibited increased ionic conductivity compared to the PS-b-PMMA-b-PS block polymer gel, as well as superior device performance in transistor gating experiments.