The synthesis of cation exchange membranes (CEMs) usually involves using organic solvents and/or a sulfonation process. In this study, green and scalable synthesis of high performance CEMs is achieved without organic solvents and sulfonation. The synthesis is carried out via in situ polymerization of lithium styrene sulfonate in a porous support. Different preparation procedures are developed and optimized. Functional sulfonate groups are successfully loaded onto and into the membrane support, as verified by FTIR. Besides, water plays an important role in membrane synthesis. By reducing the amount of water used, the ratio of functional polymers to the membrane support in the synthesized CEMs is increased. Therefore, the synthesized CEMs show increased ion exchange capacity (IEC). This is significant because it means that high IEC can be achieved without introducing cation exchange resins into the membranes. Finally, the synthesized membranes demonstrate high desalination performance. This new methodology may shed new light on preparing CEMs in an efficient and eco-friendly way.