During the operation of proton-exchange membrane fuel cells, the membrane–electrode assemblies are submitted to hydration/dehydration cycles because of the changes in temperature and relative humidity. It is therefore essential to understand the effect of these conditions on the membranes’ mechanical properties and transport mechanism. Although multiblock aromatic ionomers are promising alternative materials to Nafion, the mechanical properties during hydration/dehydration and their effect on the proton transport mechanism in these materials are not well understood. In this work, the interplay between the humidity-induced mechanical behavior and the proton transport mechanism in multiblock aromatic ionomers was systematically studied, as compared with those of Nafion. The water vapor transport mechanism in terms of sorption and diffusion was first investigated by dynamical vapor sorption. Then, the impact of hydration and dehydration on the membrane mechanical properties was elucidated using a dynamic mechanical analysis device coupled with a temperature- and humidity-controlled chamber. An innovative understanding of these polymers could be proposed by comparing the kinetics of water clustering to the consequences on the elastic properties. A major outcome is the estimation of a relative amount of isolated and clustered water molecules.