With the demands for better performance of polymer electrolyte membrane fuel cells (PEMFCs), studies on controlling the distribution of ionomers have recently gained interest. Here, we present a tunable ionomer distribution in the catalyst layer (CL) with dipropylene glycol (DPG) and water mixtures as ionomer dispersion medium. Dynamic light scattering and molecular dynamics simulation demonstrate that, by increasing the DPG content in the dispersion, the size of the ionomer aggregates in the dispersion is exponentially reduced due to the higher affinity of DPG for Nafion ionomers. The ionomer distribution of the resulting CLs dictates the dimensional feature of the ionomer dispersion. Although the ionomer distribution becomes more uniform with increasing the DPG content, an optimal power performance is obtained at a DPG content of 50 wt% regardless of feed humidity due to balanced proton and mass transports. As a guide for tuning the ionomer distribution, we suggest that the ionomer aggregates in the dispersion with a size close to that of the Pt/C aggregates form a highly connected ionomer network and maintain a porosity in the catalyst/ionomer aggregate resulting in high power performance.