Hydrocarbon-based membranes can extend the operating current range of a polymer electrolyte membrane water electrolyzer (PEMWE) because of their low gas permeability and high proton conductivity. However, increasing the ion exchange capacity for sufficiently high proton conductivity leads to poor mechanical stability and increased gas permeability for hydrocarbon-based membranes. This study presents a poly(p-phenylene)-based multiblock polymer with an oligomeric chain extender (CE-sPP-PPES) for improved mechanical stability and extended operating current range. With the introduction of a chain extender, the mechanical and chemical stability were notably enhanced. The effects of ion exchange capacity on mechanical properties, proton conductivity, and hydrogen crossover were investigated compared to a Nafion membrane. The CE-sPP-PPES-based PEMWE has a 2.1 times wider operating current range than a Nafion-based PEMWE because of its smaller ohmic overvoltage and lower gas crossover at an optimal ion exchange capacity, demonstrating the possibility of a new hydrocarbon-based membrane for PEMWE.